Upon arriving on Third Earth, the Mutants encounter a wizard by the name of Mumm-Ra who buries the Mutant Mothership beneath the desert sands as a demonstration of his power. The Mutants respect Mumm-Ra's sorcery and agree to cooperate with him to defeat the ThunderCats, but Slithe notices that Mumm-Ra has a critical limitation in that he requires time to recharge in his sarcophagus. Thus, the Mutants frequently act independently of Mumm-Ra.
The Mutants enslave a race of beings known as Brute-Men in order to construct Castle Plun-Darr and terrorize natives of Third Earth to plunder their resources, wreak havoc, and provoke the ThunderCats. However, the ThunderCats prove to be formidable adversaries and the Mutants soon revisit Mumm-Ra to request he raise their spaceship from the desert for salvage purposes to help them compete with ThunderCat technology. This enables Vultureman to develop several vehicles for the Mutants: the NoseDiver, the SkyCutters, the Flying Machine, and the Fistpounder.
Mutant varieties include: Reptilians, Monkian, Jackalmen, Vulturemen, and Ratmen.
- Slithe - leader of the Mutants; opportunistic, cunning, and skeptical
- Jackalman - assistant Mutant
- Monkian - assistant Mutant; his ability to quickly navigate the jungles helps Mutant espionage
- Vultureman - inventor, mechanic, chemist; he provides the Mutants with technological support; he's not as core to the group as the other three but he's a regularly recurring character with his own quarters in Castle Plun-Darr
- Mutant Mothership - the Mutant spaceship; salvaged to create other vehicles/technology
- NoseDiver - a hovercraft with amphibious capabilities; the favored vehicle of Slithe
- SkyCutters - light aircraft used by the Mutants
- Flying Machine - large aircraft; Vultureman's favored vehicle
- Ratstar - Ratar-O's spaceship; it crashes and is salvaged for various parts but later is restored
- Mutank - a ground vehicle created from salvaged Ratstar parts
- Fistpounder - a heavy ground vehicle created to oppose the ThunderTank; Monkian debuts this vehicle