The Lani People

The lani are discovered on a distant planet in the far future; they look human, save that they have tails. A lani with her tail removed is indistinguishable from human. The law says, however, that the key is interfertility; anything that humans can breed with is human, and anything they can't breed with is just an animal. The lani, therefore, are animals, and the story follows the adventures of the young veterinarian hired to tend a lani herd as he gradually discovers the history and true nature of the species.