Second Life is my go-to for bestiality. Most of the awful awful under-age-play crap is now underground there (while still having a thriving old/young scene), but the dog/beast stuff is pretty open.
My avatar hits the dog sims regularly, but oh my god there are just so many creeps and people who do not have any clue how sex between a woman and dog happens in RL. Like all the fake doms on SL, ugh. I think my friends list there for good male dog lovers is under 10 guys, after years playing. i try ot keep it real but some guys have no clue.
I also suspect, like the whole lesbian BDSM scene in Second Life, that
most of the women/female avatars on the dog and horse sims are guys behind the keyboard because overly sexed up avis, and not giving off female vibes in their chats. i think it's guys just playing one-handed and making beast porn with each other to masturbate to.
I told my one BFF who knows about my dog sex life that if I ever won a lottery I'd discretely finance the development of a dog sex simulator (horse DLC expansion of course for v2).
Sims4 also has bestiality mods, and the upcoming Paralives will have modding right from day one (December) with pets, so you know what'll get made