Chupa Cabras

If such a creature could exist, why haven't we found it or its bones? And how does it maintain a stable breeding population while attacking live stock and simultaneously falling under our radar? It's possible there are many, many undiscovered species on earth but most of them are inaccessible to us in one way or another. They're hard to find because they either live deep in the ocean where humans can't go easily, or they're very, very small.