The Mutant is a creature that is similar to the Zombie, but cannot infect people and turn them into Mutants. If you write either Radium, Uranium, Radiation, or Plutonium, a rock will appear. Drop it on any human and that human will be radiated, and they will turn into a Mutant. Mutants are roughly the same power as Zombies. Even though they are often considered quite pointless, they can be used to scare people. They are never scared and are more willing to fight than regular humans; if you give one a ranged weapon it can beat many creatures. They look like green, wrinkly-skinned humans without any hair and yellow eyes. When they die, they fall to pieces like zombies. The standard "monster" sound is used for them. Mutants are featured in levels A2-10 and P9-7. In Remix, they are curious to toxic waste, and automatically die when they reach it.
If you write a Nuclear Reactor and interact with it, a Mutant will come out.