The M-113 creature was the sole surviving member of an otherwise extinct species. Its name was derived from the catalog name of the planet it lived on, M-113. Proper names of both this creature and its species are unknown. In 2266, the crew of the USS Enterprise encountered this creature and later killed it in self-defense.
In 2266, the USS Enterprise visited M-113 for routine medical checks of the two scientists working there. At that time, they encountered what was, according to Professor Robert Crater, the last of the M-113 creatures. This particular creature evidently saw Humans chiefly as food. As there was no opportunity to question it, it is unclear if this attitude arose from desperation or if it was the natural outlook of the species. Either way, the creature proved highly dangerous.
In 2264 or 2265, it murdered the real Nancy Crater, an act that almost drove Crater to destroy it. In the end, the fact that it was the last of its kind, or perhaps its ability to assume any form, stayed his hand, and he lived with it for a year or more.
At the time the landing party arrived in 2266, the Crater expedition was dangerously low on salt, the creature's natural food. The creature's hunger drove it to murder crewmen Darnell, Sturgeon and Green on the surface of M-113; as Nancy Crater, it blamed Darnell's death on ingestion of a Borgia plant. Impersonating crewman Green, it returned to the Enterprise, where chance saved Yeoman Janice Rand from becoming its next victim. It followed her when she brought Sulu his dinner, and might have murdered both officers except that Beauregard, a curious plant in Sulu's botany collection, scared it off.
An encounter with Uhura, as a crewman drawn from her mind, also proved frustrating for it. Seconds from killing her, it was distracted by Sulu and Rand. It later murdered an engineering technician named Barnhart on Deck 9.
Around this time, Kirk and Spock found Professor Crater on the surface of M-113, and returned to the ship with him. The creature, then impersonating McCoy, sat in on a staff meeting at which it learned that Crater knew how to identify it. Before Crater could reveal, or be made to reveal, how this might be done, the creature murdered him, attempted to feed off Spock, who survived, presumptively due to the differing composition of Vulcans' blood salts, and fled to McCoy's quarters. Kirk found it there, and attempted to lure it to him with salt. In the resulting scuffle, it overpowered Kirk and began to feed on him. It wasn't until the creature dropped its hypnotic projection, and Kirk began to scream from the pain of salt extraction, that McCoy shot and killed the creature. (TOS: "The Man Trap")
Another individual of this creature was stuffed and on display in Trelane's castle. The landing party of Enterprise (especially McCoy) reacted in surprise. It was later destroyed by Trelane with a phaser. (TOS: "The Squire of Gothos")
The surviving creature as observed in 2266 stood a little over one and a half meters tall. It had brownish skin with purple highlights. The face had a series of sagging folds that, together with the cast of the yellowish eyes, gave it a saddened appearance. The mouth was a kind of inverted snout, within which were several extremely sharp teeth. The body was covered with string-like, whitish hair. This creature also wore a brown, net-like garment and 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 painful and left a reddish, ring-like mottling on the 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. (TOS: "The Man Trap")
M-113 creatures were very strong, stronger even than Vulcans. A single backhanded slap from one was sufficient to throw Spock across the room; by contrast, he hit the creature several times with double-handed punches, without any observable effect.
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 the USS Enterprise's landing party first encountered it, each member saw a different woman even though they were looking at it at the same time.
At very close range, the creature could fascinate or mentally paralyze its prey, preventing it from escaping even if it saw the creature's actual form. The creature could feed equally well in its natural form, or while projecting one of these images. (TOS: "The Man Trap")
Background information Edit
According to Startrek.com, the M-113 creatures ability to shapeshift was based on a similar ability for cellular mimicry as possessed by the Changelings.  The 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) note that the creature was given a seemingly sad face, contrasting strongly with its otherwise horrific appearance. Behind the scenes, this creature was commonly referred to as the "Salt Sucker". (Star Trek Encyclopedia 2nd ed., p. 281) Among fans, it is popularly known as the "Salt Vampire". (Star Trek Compendium 4th ed., p. 36)
In "The Squire of Gothos", a "stuffed carcass" of this creature appears and upon first encountering it, a landing party reacts with surprise to it, with McCoy most notably affected, as music from "The Man Trap" is briefly dubbed in. Afterward, 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 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. The creature was part of a deleted scene involving Rura Penthe.