Beavo

A beavo is a small, seemingly harmless-looking creature native to the planet Selkye that absorbs life energy from animals.

Specifics
Beavos are six to eight inches long, blue, and furry, without any legs, mouth, eyes or other external sensing organs. They move via muscle contraction like an inchworm.

They are symbiotes for an indigenous species of humanoid Selkye toad, collecting life energy as food for them, and have no independent life of their own. Leonard McCoy named the species.

History
In 2266, the starship USS Enterprise (NCC-1701) brought a Federation colonial expedition of scientists and technicians to Selkye, landing near an empty city of the long-dead Selkye natives. While exploring the metropolis, Professor Richard Spellman and his assistant Jean Ravel discovered desiccated remains of the toad humanoids along with several beavos. Leonard McCoy kept one as a pet. As the Enterprise departed Selkye and McCoy did paperwork, it clung to his arm affectionately. The next morning, however, McCoy awoke exhausted, while a corpse-like toad creature stored in sickbay had came back to life and assaulted the crew. As McCoy was treated in sickbay, Nyota Uhura took care of his beavo.

Concerned about the expedition, James T. Kirk reversed course, and upon returning to the surface, Kirk, Spock and McCoy discovered the expedition members locked within their base camp, all extremely weak, while groups of the large toad humanoids were trying to break in. Montgomery Scott could not send shuttles to rescue the scientists, because their toad humanoid had reanimated and caused damage to the ship. Uhura was also suffering from the same symptoms as McCoy. Kirk realized what was happening, and had all the beavos locked inside metal containers, which blocked the transfer of life energy to the toad humanoids.