Bestiality is sexual activity between a human and a non-human animal. It is explicitly prohibited by adherents to most religions, and the Bible says that in the case of such acts (men actually performing the act, women "presenting themselves") both human and animal should be put to death. Did the animal have any choice or any appreciation of what it was doing?
Bestiality is extremely taboo even in most secular cultures. If the animal is harmed this should certainly be illegal under animal cruelty laws.