The creatures were a highly intelligent species that used telepathy to draw in prey by extracting an image from their minds and taking that form. Once the prey was completely drawn they then used to draw salt from a prey's body through suckers that were on the creatures hands. They also possessed tremendous strength and an ability to survive several phaser shots at short range.
At the height of their existence, there were millions of creatures that roamed around the planet in a similar fashion to the buffalo of Earth. However, by the 2260s only a single specimen lived. In 2264/2265, this lone creature encountered Professor Robert Crater and his wife, Nancy. During this encounter, the creature killed Nancy and took her form to stop Professor Crater from killing it.
Encounter with the USS Enterprise Edit
A year later, the USS Enterprise arrived to conduct medical examinations of the Craters. By this time, the amount of salt contained in the Craters' encampment drove the creature's hunger to murder Crewman Scott Darnell on the surface of M-113; as Nancy Crater, it blamed Darnell's death on ingestion of a Borgia plant. The creature then went on to murder Crewmen Sturgeon and Green when a further landing party returned to the surface.
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 Hikaru Sulu his dinner in the botanical garden, and might have murdered both officers except that Beauregard, a Weeper plant, being studied by Sulu, became agitated and scared it off.
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 how this might be done, the creature murdered him, attempted to feed off Spock, and fled to McCoy's quarters, once again as Nancy. 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 telepathic projection, and Kirk began to scream from the pain of salt extraction, that McCoy was able to shoot and kill the creature. (TOS episode: "The Man Trap")
Attack on Hfihar Edit
In 2409, after a series of mysterious deaths at the Romulan mining colony on Hfihar, creatures similar to the M-113 creature were sighted in large numbers on the planet. A combined force of Starfleet and Klingon officers were able to hold them at bay long enough for the civilian population of the colony to be evacuated to safety.
Scientists observing the attacks noted that the original M-113 creature would not have been able to sustain itself on Romulan copper-based blood, and speculated that the Hfihar creatures had been genetically engineered as a weapon for use against Vulcanoid beings. Some also speculated that the appearance of the creatures had some connection to the Tal Shiar base known to exist in the system.