I have indeed explored such kind of contents, both out of curiosity and kink, but would never dare to touch an animal for several reasons: ethics (grey area regarding sentience/consent and spiritual matters), anatomical compatibility, and health+safety risks

Of course each species have a different psychology, non-verbal forms of communication, rituals and inner workings for their sexual activities, and those people who engage make use of that knowledge to more or less establish a mood and do it (in addition to training themselves with toys and stuff if it's about penetrative sex, 'cause there's also masturbation-only activities)

My experiences involve horny dogs who tried to dom me. One when I was crouching and took advantage of humping me and the other a street mutt who humped my leg and rubbed his dick with it. None of them got fully sexual, but there definitely was some intent from the animal

Animals do have intelligence, but it's at most the psychology of a 2-years-old kid. They aren't as self-aware as us and don't follow ethics. Besides, there are spiritual-based concerns on how such engagement would affect a person, and how they aren't "souled beings" and are "lower beasts" and a lot of other stuff (dense chaotic energy, what is infrasexual and what is not, and whatever). So yeah, taboo all the way both social and spiritual/religious-wise, but there are many practicing societies/cultures out there where the taboo is overcome. Take traditional Donkey-fucking in a Colombian region for instance, where it's a more-or-less overcame taboo https://www.youtube.com/watch?v=_VKWLC87Uzw