Mutant

Mutants include those born with Chaos mutations, as well as those that develop them later in life. Mutants range from individuals desperate to belong to the rest of society who conceal their deformity to the best of their ability, to those that embrace Chaos completely, join a cult, and turn all their efforts to overthrowing the Empire. Regardless of their intentions, many Mutants try to live as Humans, if their mutations will permit it. If not, they are only alive because they either fled from the authorities or they were abandoned in the woods as children and the life of an outcast is the only one they’ve ever known. Some, particularly those with the animalistic appearances, may be adopted by Beastmen warbands.

Source

 * Warhammer Fantasy RPG 2nd ED -- Old World Bestiary (pg. 102).