My favorite example would be Przewalski's horse, also known as the Mongolian wild horse. In the 1960s it was believe extinct in the wild, with a couple dozen in zoos. Zoos began a breeding program and eventually began reintroducing them to their previous habitats. Today there are something like 1,900 Przewalski's horses, and over half of those are in the wild. It's really a testament to the good that zoos can do for conservation.