Dr. Yekyll's Laboratory if you are really lucky. It's a restricted costume meaning that you can't just get it through traditional means. It was a Referral System & Facebook contest prize. Mutant pets are gray and red. Old mutant pets have yellow eyes with bags under them and their brains sticking out. Modern Mutant pets have red horns, spots, and claws. All mutant pets have deformities like extra tails, limbs, etc. This can be compared to the Monster Costume. You could catch this costume if it is put up for auction by the Halloween Alien.