Abigor

Abigor is the grizzly result of lab rat experimentation and a daring escape into a radioactive sewer system. After the modified little creature managed to escape his scientific enclosure, he fled into the sewers, of which at the time were riddled with radiation, mutating the rodent into his new, grotesque and beastly self. His brain has deteriorated, and he only speaks in unintelligible snarls whilst oozing black sludge from the mouth. 

After the monsters living on Earth were forced onto another planet, Abigor made his way into the sewer system belonging to the new planet instead.