So, this is something I have OPINIONS on.
Item 1: IRL, for either, is hard no, obviously.
Item 2: Real girls, 18+, playing with beasty toys of real or fantasy flavor, is moderately appealing.
Item 3: Fictional girls, in a text or visual medium, getting it on with a nonsapient mammal or fantasy reptile, can be hot, so long as there is some flavor of consent going on FROM THE ANIMAL. If the girl is making a move on the dog/horse/whatever, and it's not into it, that's a pass. I'm actually less concerned about the girl's consent (although my overall preferences there are CNC or reluctant acceptance, hypno, corruption, pheromones, mindcontrol, whatever, rather than actual hard r@pe.) A decent example of an edge case would be a naive schoolgirl exploring with her dog. If that gets the dog into her, I'm into it. If she has to use the peanut butter trick or other things to coerce the dog? eh, no. If the dog is pushy with her, or there's some kind of MUSK situation, I'm down for that, depending on the quality of the work.
Item 4: Fictional girls getting it on with sapient mammals/reptiles who are into or instigating the encounter? Firm yes.
Item 5: bugs, worms, infestations, and honestly, 68% of most tentacle monsters (with the exceptions being, once again, the sapient consenting type, rather than the mindless variety) = Hard no, ew.
Thank you for listening to my ted talk.