Wow! What a muta­tion! You often here of little muta­tions, like the chick­en with webbed feet but to have a duck grow an extra pair of legs!
You also get some muta­tion on humans don’t you, some of the more com­mon ones are grow­ing an extra fin­ger, and some people even grow extra toes. Although they are not proe­p­rly developed I do not believe.