Here is an article about a mutant dolphin which was caught recently that had four flippers instead of the 'normal' two. I've circled the two extra flippers in the photo to the right. The flippers look normal but other dolphins don't have them and only a few times previously have four fins been recorded on dolphins. The article claims dolphins use to be land animals millions of years ago and these fins are a left over trait from that. Additionally grow leg buds before they are born but they disappear before birth. So there you have it. Mutant dolphins for the win.
Head over and read the full article plus check out the full size photo of the mutant dolphin for yourself.