The M-113 creature was an intelligent, bipedal creature native to the planet M-113, a planet that contained, by the mid-23rd century, nothing but "the ruins of an ancient and long-dead civilization." (TOS: "The Man Trap")
This species, which once had a population in the millions, required salt to survive and so were fundamentally affected when, at some point before the 23rd century, the planet's supply of salt was lost. Due to the scarcity of the substance, the native civilization collapsed and the creature's species was driven to the brink of extinction, with the sole remaining member of the species being discovered during the 2250s. (TOS: "The Man Trap")
Another individual of this creature's species was stuffed and on display in the drawing room of Trelane's house. A landing party from the Enterprise, especially McCoy, reacted in surprise upon seeing it. It was later destroyed by Trelane with a phaser. (TOS: "The Squire of Gothos")
The last surviving creature, as observed in 2266, stood a little over one and a half meters tall. It had brownish skin with purple highlights, and yellowish eyes. The face had a series of sagging folds that, together with the cast of its eyes, gave it a saddened appearance. It had a mouth, which was a kind of inverted snout, within which were several extremely sharp teeth. The body was covered with string-like, whitish hair and wore a brown, net-like garment.
It had the proportions of a typical humanoid with two arms and two legs, each hand having three thick fingers. Each of the three fingers had three sucker-like feeding organs. The creature used these to extract salt from its prey; a process that was not only painful but also left a reddish, ring-like mottling on the victim's skin. The creature could also ingest pure salt through its mouth.
While the creature could feed on Humans, it either could not or did not wish to feed on Spock. He theorized that his copper-based blood salts were unappealing or not nourishing. Professor Crater, after his observations of the creature, noted that it was "not dangerous when fed," later mentioning that "It needs love as much as it needs salt."
M-113 creatures were also highly intelligent, capable of carrying on conversations with other intelligent beings. A form of telepathy enabled them to draw an image from the mind of someone near; this image was usually of someone trustworthy or appealing. This image, in turn, enabled the creature to approach prey easily. Furthermore, the M-113 creature could even simultaneously appear as a different image to each individual who stood in the same room. When a landing party from the Enterprise first encountered it, for instance, each member of the group saw a different woman even though they were looking at it at the same time.
Observations by Captain James T. Kirk determined that the creature was capable of attacking and slaughtering "armed and able-bodied crewmen" with relative ease. He speculated "Apparently, the killer can immobilize them as it approaches perhaps with some hypnotic or paralyzing power."
He even determined that it could assume any shape. This natural ability to take other forms was likened to how "the chameleon uses its protective coloring, an ability retained no doubt from its primitive state" in order "to stay alive." Over time, Crater had learned to see it in whatever form it became.
Physically, M-113 creatures were very strong, stronger even than Vulcans. A single backhanded slap from one was sufficient to throw Spock across a room; by contrast, he hit the creature several times with double-handed punches, without any observable effect. (TOS: "The Man Trap")
Background information Edit
The invention of this alien was inspired by "The Man Trap" writer George Clayton Johnson having recently written "All of Us Are Dying", an episode of The Twilight Zone that features a con man who can shape-shift his face. "So, that transformation thing was still sort of fresh in my head, and I think almost every writer tries to get some more mileage out of anything that works," Johnson reckoned. "So that was, I think, why I thought of the idea of trying to do a story about a creature that could appear to be anyone." The idea was approved by Gene Roddenberry before the character (and the story) was fleshed out. (The Star Trek Interview Book, p. 136)
Initially, the look of the M-113 creature was inspired by the Orion slave girl version of Vina from unsold Star Trek pilot "The Cage". In a memo from Robert Justman to Gene Roddenberry (dated 15 April 1966), Justman commented, "I don't know what the grotesque biped should look like, or how we are to go about handling this creation. I might make a suggestion which goes in an entirely opposite direction. Perhaps the grotesque biped could be an extremely beautiful, but terrifying, young lady. Perhaps she could be something along the lines of the green dancing maiden we had in Star Trek I. Would you believe a blue dancing maiden? With orange hair? And plenty stacked." Justman concluded the memo by saying he'd like to be involved in the casting of the role.
The presentment of the M-113 creature in a revised story outline for "The Man Trap" impressed NBC. In a memo he wrote Gene Roddenberry (on 19 April 1966), NBC Manager of Film Programming Stanley Robertson commented, "The idea of introducing an animal as a central antagonist in one of our stories is excellent. We would suggest that the writer do some research regarding the availability of any projections or prognostications by recognized authorities into the physical descriptions of 'animals' who might inhabit planets other than our own. As are the 'educated guesses' of the types of human life which might inhabit other 'worlds', this material might be fascinating and helpful. We would caution against making the animal so 'far out' that its effectiveness is not believable."
As originally scripted by George Clayton Johnson, the creature didn't come aboard the Enterprise until the third act of "The Man Trap", considerably later than when it arrives on the vessel in the final version of the episode. This change in the portrayal of the character was suggested to Johnson by John D.F. Black. (The Star Trek Interview Book, pp. 136-137; Cinefantastique, Vol. 27, No. 11/12, p. 101) "He said, 'Oh, you see the problem is you don't get the creature aboard the ship fast enough. The danger is once the creature gets aboard,'" remembered Johnson. (Cinefantastique, Vol. 27, No. 11/12, p. 101)
Also, in the climax of the first draft script, the creature ate Professor Crater, killing him. The conclusion of the episode also included an explanation as to why the creature hadn't attacked McCoy.
Following the submission of the first draft script of "The Man Trap" (which meanwhile had the working title "Damsel with a Dulcimer"), Robert Justman sent John D.F. Black a memo (on 24 May 1966) in which Justman admitted that, when reading certain scenes from the script, he was unsure whether the creature was disguised as "young Nancy or older Nancy." Justman also suggested a cutaway for the transformation from Nancy Crater to the creature's natural appearance. "I also feel we need a discussion as to what the creature should look like and what it should do," the memo continued. Justman went on to express uncertainty over why the creature ate Crater alive, especially because it had consumed a provision of salt tablets shortly prior to that scene.
In a scene breakdown of the story (which was undated, though the narrative was meanwhile titled "Damsel with a Dulcimer"), the creature's real appearance was described as "a strange creature with the body of a young woman and hands like the legendary hair of Medusa – a clump of snake-like tentacles at the end of each wrist."
In the second draft script of "The Man Trap" (which was still called "Damsel with a Dulcimer"), the M-113 creature's extinct civilization was cited as being responsible for the archaeological remains on the planet's surface. In a scene from the same script, the last of the creatures, while disguised as McCoy, was watched on the ship's bridge, by Kirk and Spock, as it made its way towards the real McCoy, who was meanwhile asleep.
In another memo Robert Justman wrote John D.F. Black (on 2 June 1966), Justman commented, "I understand that the various guises that Nancy appears in are to key off of whosoever is looking at her. And we should follow this method throughout the script. However, what does she look like when Crater looks at her?" Much like Gene Roddenberry, Justman also expressed puzzlement over which version of "Nancy" would appear in which scene. As the memo went on, he advised, "We will have to discuss with Gene our conception of what the Creature really looks like and how Nancy changes into this Creature. Also, how are we to create this Creature. The various visualizations of what the Creature is cannot dissolve into each other, but will have to be some sort of an optical ZAP EFFECT due to the difference in physical characteristics [....] Some time at our leisure when we can sit down and discuss this story, I have some ideas to bring up with regard to the Creature and the forms it assumes in the mind's eye of other people."
In a memo Gene Roddenberry wrote George Clayton Johnson (also on 2 June 1966), Roddenberry stated, "I think we should specify very clearly in script for the benefit of the director that when we go from one Nancy to the other, we will go from a highly identifiable pose, movement, or something so that the audience continually understands that this is the same woman seen with two different sets of eyes. For example, if she is holding an object, then we go to the other one and see her holding the same object against the same background, the thing will be much clearer. Also, we should probably do the switch in mid-sentence occasionally too. (Incidentally, one of the two will have to be looped since maintaining the same voice will be very helpful in keeping the situation clear.)" Roddenberry also felt the M-113 creature's society, which he described as "a lost and strange civilization," should be more archaeologically evident on the surface of the planet. "Were they bipeds? [....] I presume it is an animal which originally attracted its prey via hypnotic vision," Roddenberry mused. "Or, if it's an intelligent creature, perhaps that ability is one left over from its animal past, just as certain of our fangs and muscles are left." According to George Clayton Johnson (The Star Trek Interview Book, p. 139), Roddenberry particularly liked the last of the creatures being linked to "the last of the buffalo."
In a memo Robert Justman wrote John D.F. Black about the revised second draft script of "The Man Trap" (the memo was dated 10 June 1966), Justman commented, "I still feel there is an enormous problem concerned with what Nancy looks like when certain people are looking at her. And I don't think that this problem has been resolved yet. Especially at the end of the story, when Kirk and Spock and McCoy are in the throes of getting rid of Nancy, or whoever she is. How does Spock see her? Are we going to have to keep on jumping two or three different actresses back and forth within the same sequence, depending upon who is looking at her? If we do this, we can be shooting this show forever, because we will have to cover it for all looks." Justman went on to propose that "Nancy" be seen the same way by Kirk, Spock, and McCoy, whereas Darnell, as was already established in the script, saw her as a brassy blonde. In Justman's opinion, it was believable that "Nancy" be seen the same way by Kirk and McCoy, as they had evidently discussed her before meeting "Nancy", and Justman believed Kirk would already be aware of how she looked.
In the final draft script of "The Man Trap" (dated 16 June 1966), the second page of that document stated:
"NOTE TO DIRECTOR: Throughout the script we will see 'Nancy' as various different people. Crater and McCoy see her the same because both knew the real Nancy. Others see what they want to see or what they expect to see. 'Nancy's' ability to change her appearance is a form of protective camouflage. Just as a chameleon can change its color to blend with its surroundings, so can 'Nancy' change her physical appearance. For clarity so that the audience will be able to follow the transformations as they take place, we suggest you find a highly identifiable pose, gesture or movement that will be characteristic of 'Nancy'... possibly seeing her with folded hands – hands clutched back to palm – a sort of hand washing gesture that is highly individualistic without being stagey." 
The final draft of the script additionally contained a bracketed director's note regarding the moment when it becomes evident that McCoy and Kirk are seeing the creature as two different versions of Nancy Crater. The note stipulated, "Here, as always when transforming her identity, always cut on highly identifiable action, gesture, or mid-speech." However, no such characteristic is shown in the final version of that scene. In an ultimately unused line of dialogue from the same script, the M-113 creatures were established as having multiple "temples" on the planet. However, the only physical description of the creature in the final draft script was that it had "suction-cup tentacles."
The M-113 creature was designed and built by Wah Chang. (Star Trek Encyclopedia (2nd ed., p. 281)) The authors of Star Trek: The Original Series 365 (p. 047) noted that the creature was given a seemingly sad face, contrasting strongly with its otherwise horrific appearance.
Sandra Gimpel, a dancer and occasional stuntwoman who had previously doubled for a Talosian in "The Cage", received a call inviting her to play the creature in "The Man Trap". She was sought for the part because the staff "wanted somebody small, and they knew that I could do costume work, because I'd already worked for them." 
Sandra Gimpel subsequently went to the studio for costume fittings. "We had to make sure that thing fit, and that I could move into it," she said. "Then the arms were longer than my arms, because of all the suction cups." The ugliness of the costume was immediately evident to Gimpel, though she forgot how effective the costume was while wearing it. "It was kind of funny to me," she stated. "I enjoyed all that crazy stuff anyway, so… [....] The hard part about the costume was you couldn't see, because you had no peripheral vision, and then you couldn't look down. When you're walking towards your mark, you couldn't see your mark on the floor, where to stop, because the nose was in the way [....] [The costume] was hot and heavy. But the nice thing about that particular costume was you could take the head off." 
Having studied acting at college, Sandra Gimpel found that playing the creature, in collaboration with Director Marc Daniels, was generally very easy. "When he started telling me what to do, I didn't have any troubles," she related. "I could listen and do what he said. We rehearsed two or three times, to make sure I could hit my mark in costume. We did it without the head on, so I knew where I was going. And it worked out fine." One of the scenes, where the creature extensively touches Kirk's head, was somewhat difficult for Gimpel to play. This was because, although William Shatner was, in Gimpel's words, "a star", she had to use the suction cups to touch all over his face. Gimpel was also required to portray the creature's death. "They said, 'Can you fall down?' I went, 'Sure!' So I did the hit against the wall," she recalled. 
In the script of "The Squire of Gothos", the M-113 creature was consistently described as a "lizard-like creature", with no indication that it was to be a reuse of the M-113 creature suit from "The Man Trap".
Following its appearance in "The Squire of Gothos", the "creature" (along with the Gorn and some other "monsters" of the series) found a new home in Robert Justman's office. (Inside Star Trek: The Real Story paperback ed., p. 215) In Cinefantastique (Vol. 27, No. 11/12, p. 34), the creature was incorrectly referred to as having reappeared in "Catspaw" (instead of "The Squire of Gothos"), and the costume was said to have subsequently ended up as a permanent fixture of Gene Roddenberry's office.
In an interview in 1998, Michael Westmore said he would have liked to use the M-113 creature in an episode of Star Trek: Deep Space Nine or Star Trek: Voyager, but this never came to fruition. (Star Trek Monthly issue 46, p. 83)
Barney Burman and his company Proteus Make-up FX Team created a "salt sucker" alien as an homage to the M-113 creature for J.J. Abrams' Star Trek, in which the alien was intended to be shown in a scene set on Rura Penthe, though the scene was ultimately cut. At least one conceptual sketch of the creature was illustrated. The alien was also one of two sculptures which the movie's key sculptor, Don Lanning, worked on whenever he got some down time while assigned to the film, the other being the Gorn. "The Salt Vampire was a real challenge," Lanning recalled, "to take something that was maybe a bit hokey and turn it into something really organic, but we certainly gave it a real try. In the end, it never made it into the movie, but to tell you the truth, I'm kind of glad." (Star Trek Magazine Special 2014, p. 137)
For fifty years since playing the creature in "The Man Trap", Sandra Gimpel was completely oblivious to the creature's popularity. Although she was interviewed multiple times during the intervening years, the interviewers never asked her about the creature. "I just never thought about it, you know? [....] So I didn't understand that it was that crazy monster that they loved so much," Gimpel explained. It wasn't until she attended her first Star Trek convention, in 2016, that Gimpel finally realized how popular the character was, which she was awed and thrilled by. When interviewed at that convention, she remarked, "Ninety percent of the people that walk up to me tell me that that was the scariest thing they'd ever seen, that they couldn't sleep at night. And I'm like, 'OK' [....] It may sound silly, but I'm in awe of the fact that everybody loves me so much, and they come up to me and say, 'Thank you for coming.' I'm like, 'Thank you for having me.'" Gimpel was also wowed by being told about the presence, at various other events, of cosplayers who dressed up as the creature, and by a cake, at the convention she attended, that looked like the alien. 
According to StarTrek.com, the M-113 creature's ability to shapeshift was based on a similar ability for cellular mimicry as possessed by the Changelings.  The M-113 creature is also somewhat similar to an alien species called the Coeurl, who appear in literary science fiction works by A.E. van Vogt, including the novel The Voyage of the Space Beagle. The Coeurl suck potassium ("id") from their victims, much like how the M-113 creature extracted salt from its victims. Noted George Clayton Johnson, "I always think of the Space Beagle as that hostile creature that's prowling the ship." (The Star Trek Interview Book, p. 141)
In the Star Trek Online mission "Mine Trap" (a reference to the TOS episode title "The Man Trap"), players must cooperate to defend Romulan colonists from an invasion of M-113 creatures while an evacuation is arranged. The creatures are resistant to handheld weapons fire, can change their appearance to infiltrate crowds of colonists, and use psionic attacks against their enemies.