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. 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. 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.
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. As part of an associated ritual the mahko root was pressed against the victim's wound, convulsed of its own power, and neutralized the toxin. (TOS: "A Private Little War")
The Mugato was called the Gumato in the original 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 Demon 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 an episode of Here's Lucy. In the end credits of "A Private Little War", Prohaska receives credit as Gumato.