The Mercaptans were a fictional race used in security and tactical problems at Starfleet Academy, such as in the Advanced Security Techniques course. They were completely hostile and consisted largely of nameless, faceless warriors.
They changed and evolved as each new class of cadets added to their characteristics, growing more aggressive and increasingly eccentric.
By circa 2358, a Mercaptan stood three meters high, had both scales and fur, and was armed with claws, fangs and hand-held photon torpedoes. They had no king but were led by a "Troop Controller", who had a personal force field. Mercaptan warriors protected their Troop Controller at all costs, and went berserk without his guidance, attacking all in range, even each other.
At that time, Darryl Adin and Tasha Yar considered a tactical exercise in which they had to seize a hill guarded by seventeen Mercaptans using only three security personnel, including Thonis. (TNG novel: Survivors)