There are lots of animals still around that are remarkably unchanged from millions of years ago. These are popularly called "living fossils," though that phrase is not usually used by scientists, and often give us insight as to the basal, or primitive forms and evolutionary history of other, more recently evolved animals. They also give us significant insight into the geologic history of the planet; animals survive when they can reproduce at a stable rate, simple as that. But these animals plateaued early, finding a way to breed, often in isolation (on islands, for example), at a stable rate, and remained successful without changing much, physically. We don't really understand why this happens to some animals and not others, but the facts are pretty clear: they were doing fine, so they pretty much just stayed the way they are.