They are failed experiments of Professor Monkey-For-A-Head, and the Professor unleashes the mutants upon Earthworm Jim in repeated efforts to reclaim the Super Suit which the Professor designed, and which Jim is wearing.
They are enlarged, still-living human brains, given the ability of movement by the Professor, and have the tail of a rat sewn onto their rear portion. They also apparently have the senses of a rat, being able to detect Jim from a distance. Although easily dispatched with Jim's Plasma Blaster, they often hunt Jim in groups. The mutant brains are particularly agile and have an impressive leap, able to reach high places and across platforms, often giving Jim a nasty surprise.