The Shulchan Aruch writes in hilchos treifos that if an animal has an extra leg it is a treifa, as there is a rule that anything extra is as if one of the originals was missing. if that limb or organ missing would make the animal a treifa, so with an extra one it is also a treifa. An extra leg therefore makes the animal into a treifa. the Shulchan Aruch qualifies that by saying this is only correct if the extra leg is connected in a certain way, at the top of the lower bone, the way a normal leg is connected. Without that specific connection, it would not be considered an extra leg and would be kosher.

The Rama adds that the leg must be in line with the other legs for it to be considered a leg and therefore a problem. Without that, the leg would not be a leg but would just be an appendage (that happens to look like a leg) and would be kosher.

When you learn shulchan aruch you learn about a lot of things that sound like they probably never happen, and are hardly relevant. Yet they still once in a while surprise us and come up in real life.

4-legged chicken

Yoelish Krause, of being the "operations officer" of the Eida fame, hosts a public session of schechting chickens in his courtyard once a week. Yesterday, as the shochtim were going through the chickens and slaughtering them, they pulled out one of the chickens only to find that it had four legs!

You can learn about such thigns as much as you want, but because of the rarity of ti actually happening, when it does happen you are not going to just look at it and make a quick decision, but will show it around, take it to different rabbonim, have it inspected and hear different opinions.

And that is what Krause did. He took it from kollel to kollel to display it and generate discussion on whether this chicken was kosher or treif. One of the kollels was that of Rav Yaakov Yosef who paskened that it is a treifa, though if after slaughtering it and opening it up it would be discovered that the leg is connected differently it would be kosher. Rav Tzvi Friedman on the other hand inspected it and said it is kosher as the legs are not connected in a way that would deem them as extra legs..
(sources: Kikar and Bechadrei)