I put a lot of gray whales back into the Atlantic ocean. Which wasn't so terrible. But then I got cocky and added in the Rodhocetus which has a somewhat more particular appetite, and things went belly-up fast. That was before anyone had told me I could do things.
Unfortunately if you start bringing things back, extinction doesn't mean much, does it? If you destroy an animal, seems to me the thing you need to learn how to deal with is a dead animal rather than a magical fix-it.