The whole "no bestiality" thing strictly refers to real-world animals. While I'm not sure about extinct animals (extinct animals such as dinosaurs are allowed on platforms like Amazon because you can't "encourage sexual violence" against animals that are extinct), fantastical creatures are exempt from this for the most part. However, it does get kind of murky when it comes to fantasy races that are heavily inspired by real-world animals. For example, centaurs and werewolves (in their werewolf forms) aren't allowed on something like Amazon because they have animal parts based off of real-world animals. However, dragons (which could be argued are similar to reptiles) are fantastical enough to be separated from any real-world equivalent. A horse is clearly related to a horse. A werewolf is clearly related to a wolf. A mermaid, while fish-inspired, is probably human and distinct enough from being actually like any fish that it would be allowed on something like Amazon with no problem (and if it's safe by Amazon standards, it's definitely safe here).
Basically, if you're ever worried about "bestiality" when it comes to fantasy races, just ask yourself if it is directly related to any real-world animal that is not extinct. If it only shares some visual similarities (dragons and reptiles, mermaids and fish), you're 99% fine. If it is directly linked to a specific animal (centaurs and horses, werewolves and wolves), proceed with caution. If it's a real-world animal that is now extinct, proceed with even more caution. If it
is a real-world animal that is not extinct, congratulations you've got bestiality and you're not going to be allowed to write that.
Also, never forget about the Harkness Test.
Sorry to bump it, but it says content that encourages those illegal sex acts
In many cases, going into explicit detail is seen as encouraging it. The logic on many platforms is that, if you're describing it rather than fading to black, you're "encouraging" it. Things like pedophilia, incest, bestiality, etc are all allowed as long as you're not encouraging it typically. You would just fade to black or imply that it happened rather than write a whole chapter describing it. At that point, you're including it for the story rather than sexual gratification which is generally considered okay. Of course, there's a massive difference between writing something in explicit detail and actually encouraging others to go out and do what you're writing, but that's how most companies lazily figure out whether you're encouraging something or not.
Oh, wait, I just realized that most of this thread is pretty old. I'll remove the older quotes