The Pakleds were a humanoid species in contact with the Federation by the 2360s. To the casual observer, the Pakleds appeared to be intellectually challenged, especially in their verbal skills. However, despite the marked lack of intellect, they proved to be far more cunning than they appeared, and used their innocuous appearance to deceive other races.
The Pakleds' own technology was significantly less advanced than that of many other spacefaring races. According to Data in 2365, it remained "a genuine curiosity how they ever mastered the rudiments of space travel." They were driven by their unwillingness to develop technological knowledge on their own, desiring to instantly gain the power that other cultures enjoyed. They therefore tried to acquire technology from other species, generally by theft. (TNG: "Samaritan Snare")
In 2365, a Pakled transport named Mondor sent out a Mayday in the Rhomboid Dronegar sector 006 in order to lure in other ships whose technology could then be stolen. They were offered assistance by the crew of the USS Enterprise-D, who lent the Pakleds their chief engineer, Geordi La Forge, to help fix the Pakled ship. The Pakleds kidnapped La Forge, but eventually returned him, after the Enterprise demonstrated its "force" with a harmless pyrotechnic display involving the Bussard collectors. (TNG: "Samaritan Snare")
List of Pakleds
The Pakleds were created by "Samaritan Snare" writer Robert L. McCullough. He thought them up in the knowledge that, no matter how bizarre the aliens were, the TNG production staff would most likely be able to find some way to portray them. "I wanted to do something fun. In all these space shows, the aliens are so damn smart and sophisticated, I thought, 'How about having them come across as dummies?' As all the aliens were malevolent and intuitive like Q, I felt it would be neat to find some aliens left out of the loop of development," McCullough laughed. "Third world aliens! They appear to be stupid, but they're not: they're just needy. They're so needy, they become desperate." He was extremely pleased to create the "dull, laughable and grossly overweight aliens." (Starlog, issue 187, p. 54)
Robert McCullough took a logical approach to naming the aliens. "They were called Pakleds, which is pretty obvious – they were sheep," he noted. Although only two members of the species were named (both of them in "Samaritan Snare"), their names – Grebnedlog and Reganod – also made some sense. "These were names I learned in junior high school," McCullough explained. "My last name used to be Goldenberg and my best friend's was Donager, and we had a code of calling each other by our last names backwards." (Starlog, issue 187, p. 54)
The script of "Samaritan Snare" described the Pakleds as "slothful, droopy-eyed humanoids" with "typically wistful Pakled hypoplastic speech, almost a sense of hopeful longing in their every word." 
When designing the Pakled appearance, the make-up department had to bear in mind that the Pakleds were to be introduced as a race of aliens intended to appear slow-witted and harmless, incapable of creating their own technology, but in reality were a sharp and nefarious race who would steal anything they wished. To aid the make-up department in representing the aliens' dual nature, the casting department hired portly actors whose expressions were naturally benign, in order to convey a sense of innocence. (Star Trek: Aliens & Artifacts, pp. 111 & 113) Robert McCullough commented, "They got the biggest actors they could find, and with those costumes, they looked huge." (Starlog, issue 187, p. 54) The make-up itself consisted of appliances for the foreheads, cheeks, and a pointed lower lip. The make-up department also came up with a set of eyebrows that went up to the center of their foreheads, intended to project a sense of helplessness about their situation. For the mouth, a set of double-size teeth was used. (Star Trek: Aliens & Artifacts, p. 113)
"Samaritan Snare" Director Les Landau was careful to show the Pakleds in a balanced way. Of their appearance in the episode, he recalled, "What I had to deal with was a race of people who appeared to be dimwitted and unattractive, but yet had a need for things to make themselves sufficient. I didn't want them to appear silly, foolish or unrealistic, or rather unbelievable, so it was a constant self-conscious awareness to keep them threatening yet simple." (The Official Star Trek: The Next Generation Magazine issue 21, p. 42)
According to an internal reference document, listing all the aliens seen in Star Trek: The Next Generation, which was circulated around the time of the production of TNG Season 5, the Pakleds were given the description of "A rotund, lethargic scavenger race."
Though their only major appearance was in the TNG Season 2 episode "Samaritan Snare", the Pakleds were also mentioned in the TNG Season 4 installment "Brothers" as having rescued Data's brother, Lore. The Pakleds were originally to have been led by him in the sixth season two-parter "Descent" and "Descent, Part II", though they were ultimately replaced with the Borg in that pair of episodes. (TV Zone, issue 61, p. 38) Subsequently, the Pakleds went on to appear in the background of several Star Trek: Deep Space Nine installments. A person wearing a Pakled costume can be seen in a bar on Ronara Prime in TNG Season 7 outing "Preemptive Strike"; however, he lacks the unique Pakled facial features.
The Pakleds were briefly considered to feature as main antagonists in DS9: "One Little Ship", when that episode was in pre-production, by which time it was too late to make such a drastic change to the episode. "During prep I started saying, 'We should have used someone like the Pakleds from TNG,'" offered René Echevarria. This idea was dismissed by Executive Producer Ira Steven Behr in favor of Harcourt Mudd instead being used as the villain, despite neither of them ultimately being used. (Star Trek: Deep Space Nine Companion, p. 531)
In the script for Star Trek Nemesis, the android B-4 revealed, in a lengthy speech, that he had told a group of Talosians that he originally had been rescued from his homeworld by the Pakleds, whom he referred to as "fat," prior to him having ended up in space for a long duration and then being found by the Talosians. In the first draft script of Nemesis, B-4 outrightly informed Data that he had been rescued by the Pakleds and had later relayed news of the rescue to the Talosians, though, in the shooting script, B-4 phrased it by saying only that the Pakleds' involvement had been what he had told the Talosians. Also, in the first draft script, he mentioned that the Pakleds had traded him to a Bolian ship, which he didn't say in the shooting script. 
In the Strange New Worlds 10 short story "A Dish Served Cold", set in 2373, a Borg cube en route to Sector 001 encounters a Pakled freighter, but the Collective considers them unintelligent and an inefficient use of resources to assimilate. However, they are later assimilated when the Borg need extra reinforcements in their battle with Starfleet forces in Earth orbit. They are designated as Species 95012.
In Star Trek Online, Pakleds are a playable race for Starfleet, which may indicate they have become a member of the Federation. Visually, the Pakleds are a lot less bulky than in Star Trek canon (though their appearance can be modified in the character customization editor).
Pakleds are also one of the races that can be discovered in the video game Star Trek: The Next Generation - Birth of the Federation.