After some experimentation with human males in my younger days, playing as both pitcher and catcher, I discovered I really had no serious interest there when compared to sex with a female human, so I call myself straight with humans.
Critters: I like putting "it" in, and having "it" put in me - within physical compatibility limits - I don't have, nor do I fancy having, a size 27 asshole, thanks, and I'm not about to consider boning anything that's much smaller than a border collie. Beyond that, it's about them being willing (if not actively soliciting) and whether or not they hold any attraction for me.
Dogs: Feel free to bury your bone in me if you're capable of reaching the hole - Don't hesitate to tie if you're able. I also dearly love being balls deep inside a bitch - Unless she requests anal (and yes, I've had bitches who did exactly that - usually when out of heat when I asked if we could play), and convinces me she's serious, I prefer slipping into the hole that's designed to be filled with a penis, thanks. Only twice have I met male dogs that liked and would actively request anal penetration, though I've been with at least three who just plain didn't care, so long as they were in the squirting phase of an erection when the penetration occurred. The majority, though, said "NO!", and that was that. <shrug> So be it.
Horses: I'm a lifelong horseman, and particularly prefer mares, though I will attempt (successfully a few times, but usually not) to bring a stallion off by hand/mouth if he makes it known he'd like some sex. No, I haven't, and I never will, let even a mini cover me, let alone anything bigger. Others might, and that's their lookout. Not this guy, thanks.
Others: Depends on my personal level of attraction and accessibility - too many to list individually, but depending on various factors, I might get with just about any willing critter of either sex, either as pitcher or catcher, should the opportunity present itself, if it's a species I find sexually appealing and judge to be dimensionally compatible, and there's not a significant risk of injury or death (which leaves out big cats and gorillas, ferinstance) from the attempt.