The Creeps are a widespread and dangerous gang of mutants. Unlike other mutants, who rely on crude, or no weaponry at all, Creeps are intelligent enough to use firearms, and tactics. These little freaks will scamper around and take pot shots at their foes.
Creeps have a strong dislike of humans, and will mutilate any they see.
The original Creeps were mutated convicts from Stygia, who were exposed to gamma radiation, due to the awful working conditions they were forced to deal with. The Skulls, a rival gang, treated the Creeps like cattle, due to a lack of food, so the Creeps gutted any human they saw as payback.
It's unknown where the newer Creeps are coming from, but possibly some survivors may have been exposed to the same level of Gamma radiation as was the original batch.