In the mythological world not all members of a race are created equally. somtimes a creature is born with extraordinary powers or somthing happens that changes it forever. good or bad there are individuals that besides one or two things are exactly like an average member of the creatures race. these creatures are entirely up to the imagination of the DM. mongrelmen and formorian would go under the 'mutant' category while a lightning bat, giant gerbil, or a bull with flesh like steel and breath that can petrify would fit under 'superpowered'
Listed here are more famous examples of creatures with superpowers and mutations: