Bestiality

There is an article on Wikipedia on this subject under its formal name, Zoophilia.

Bestiality refers to the paraphilia of human/animal sexual activity.

Having the animal as submissive is only rarely considered a part of BDSM activities but a significant minority of those who associate with BDSM occasionally do so in a context submissive to an animal. In many countries, any sexual activity involving animals is illegal and in most countries it is a taboo subject.

Bestiality is distinct from the activitiy of being treated as an animal. Some submissives enjoy being treated as - or even living as - an animal, especially a canine or pony.