Mutants are creatures that have appeared all around US after the war.
Little is known of them in scientific term but there are several theories trying to explain thei appearance. None of them has been proved or explains all aspects of mutant presence.
Whatever the origin, mutants seem to be extremely well adapted to the new world. They are tough, strong, resilient to environmental conditions and some of them can even regenerate. Combined with ferocious aggresion and, in same cases, human intelligence they make worthy opponents.