Honestly, it doesn't matter. Anybody interested in seeing what actually happens between humans and animals can find it pretty much with any search. Even though a lot of it is commercial and staged, it will still satisfy basic curiosity. But there aren't any really good artistic movies.
I could suggest something artistic in terms of normal porn from the 70s (almost anything by Radley Metzger, to start). But nothing like that has ever been made involving animals.
The most real thing I've seen has perhaps been Mona and Lisa, and that's only a small part of what's possible.