According to Jack Ransom, at some point prior to 2380, "horned gorillas, sentient tar, [and] spores that make you hook up with your best friend's sister" were all types of disasters that he had personally experienced on away missions that should have been a walk in the park. (LD: "Temporal Edict")
Mugatos were large, intimidating animals that could reach a height of two meters (not including their horns). Covered by a thick pelt of white fur all over their bodies with the exception of their faces and hands, they were similar to the great apes of Earth in their physical proportions and prehensile hands and feet. Mugatos had large, thick horns projecting from the top of their craniums as well as smaller spikes running down the spine.
Mugatos were known to attack humanoids. The Mugato would wrestle a person to the ground in order to bite it and inject its poison. All of their teeth were sharp and serrated and their fangs contained a strong venom that was fatal to Humans and inhabitants of Neural alike within a matter of hours. Mugato venom was a toxin that attacked a humanoid's nervous system. Death was slow and could take several hours. The only known cure as of 2268 was the use of the mahko root by a female shaman of Neural's Hill People known as a Kahn-ut-tu.
Originally called a Neuralese Great Ape in the first draft script, dated 7 August 1967, the Mugato finally became the Gumato in the later drafts script; however, DeForest Kelley could not pronounce the name correctly and it was changed. Both Kelley and Shatner pronounced the name as Mugatu all the way through the episode, and Mugato is still frequently misspelled Mugatu in various sources such as The Star Trek Compendium.
In the film serial Flash Gordon, the native Orangopoids from the planet Mongo resemble the Mugato. These creatures were colloquially known as "The Dreaded Beast of Mongo," where they were often used in ritualistic executions of so-called enemies of the state.
It also bears some minor resemblance to the gremlin from the "Nightmare at 20,000 Feet" episode of The Twilight Zone. William Shatner starred in this episode as a neurotic airline passenger. The creature terrorized Shatner's character until he went mad.
The Mugato costume was created and performed by Janos Prohaska. It is believed that Prohaska also used the costume for the "Fatal Cargo" episode of Voyage to the Bottom of the Sea, without its head horn or back spikes (DVD season 4, volume 1), and in Season 1, Episode 22, "Lucy's Safari", of Here's Lucy. The Star Trek Encyclopedia (4th ed., vol. 2, p. 57) noted Prohaska had designed and built the mugato from an ape suit he had previously built for another project.
In the end credits of "A Private Little War", Prohaska receives credit as Gumato.
The novel series Star Trek: New Frontier featured an augmented Mugato named Janos (named after the original Mugato's performer Janos Prohaska) who was genetically-altered by Doctor Marius Bethom and achieved sentience. However, his augmented genes led to an irreversible decrease in intelligence that came in waves before becoming permanent. He entered Starfleet with the rank of ensign and served as a security officer aboard the USS Excalibur and USS Excalibur-A. In 2376, he was temporarily transferred to the USS Trident where he had a short-term relationship with M'Ress. However, his intelligence finally deteriorated, leading him to murder fellow crewmember Lieutenant Gleau after he mentally assaulted M'Ress. Having lost his unique sentient abilities, Janos was eventually brought to Neural and released back into the wild where he joined a tribe of Mugato. He eventually got into a scuffle with several members of the tribe after defeating at least two members using Judo throws implying that while Janos lost his intelligence, his muscle memory remained un-impaired.
The Star Fleet Medical Reference Manual lists what to do in the event of a Mugato bite in the first aid section and lists the Mugato's taxonomic nomenclature as Pongidae theropoda and that it can reach up to 2.2 meters in height.