When an animal's genes change, or mutate, the new form of the animal that results is a mutant. One example of such a mutant is a blue lobster. Another is the teenage mutant ninja turtle.
You can use the scientific term mutant for any plant or animal that's the result of a change, or mutation, in the DNA of that organism. When the word is used as an adjective, it describes something having to do with this kind of mutation, like a mutant chromosome or a mutant lobster. Mutant stems from the Latin mutantem, which means "changing."