The mutants are natives of the planet Plun-darr, and sworn enemies of the ThunderCats.  When Thundera was evacuated, a group of mutants followed the last ships to leave, destroying all but the flagship which carried the legendary Sword of Omens.  

A brief battle ensued on the ship, but the mutants were defeated by the ThunderCats.  Deciding not to give up, the mutants followed the flagship to Third Earth, and have been attempting to steal the Sword of Omens ever since.  Their base is the Castle Plun-darr, a fierce looking shelter and command post.

S-s-slithe is the leader of the mutants.  He is a reptilian with a quick temper.  Even though he has had difficulty accomplishing goals, he does have a good plan once in a while.  Although he has used a variety of weapons, his most useful tends to be his large tail.

Jackalman is a decent fighter, but he's been known to tuck tail and run when he knows he's outnumbered.  Wishing to be a leader himself, he once was general of a small, short lived army.

As a Simian, Monkian's greatest ability is being able to climb trees and jump from limb to limb with ease.  His battle cry is a loud and somewhat frightening, and he seems to be a believer of the use of brute force.  Monkian's weapon of choice is the footman's mace, and he carries a special shield with an ape's face on the front which can shoot energy at enemies.

Rata-O was a character added later, who came to Third Earth in his own vehicle, the Rat Star.  Cunning and shifty,  Ratar-O especially likes to pick on snarves.  His special weapons are the Rat's Eyes, a pair of sai (three pronged swords).

Vultureman joined the series when it was already in progress, but it was never mentioned exactly where he came from.  An inventor, Vultureman also has a vehicle of his own, named only as 'the flying machine'.  He is also credited with the creation of many devices, most of which did not work properly, or did not work at all.