You are confusing the definition of a category with the realization of category members. We say that a horse has four legs and two eyes, but horses are born wit

You are confusing the definition of a category with the realization of category members. We say that a horse has four legs and two eyes, but horses are born with three legs sometimes, or get into an accident and lose a leg, or they are born blind, etc. That doesn't mean they stop being horses or that we invent a new species of horses that have only three legs. Similarly the existence of Siamese twins doesn't mean we need to rewrite classifications of what an adult human male is, etc. That's not how categorization works.