Greg responds...

There may be fauna on Avalon. And the magic of the place may have had some small effect on them. Like sorcerous radiation.

But fauna would not have attended the Gathering. So any seeming beast you saw there, like Anansi for example, is one of the Children in a form of his or her choosing. (If you see a polar bear walking around the palace, the odds are it's Odin.)

Now Slepnir is another story. If the legends are true, then Slepnir's mother was the trickster Loki, and his father was an actual horse. Making Slepnir half-horse and half-fey. (Which might serve to explain his modern transition from eight legs to four.) I haven't decided 100% if that's the route I'm taking in the Gargoyles universe, but the notion is appealing.

And it would suggest that New Olympus is filled with all sorts of bizarre beasts who are the descendents of various unions between the fey and so-called lower animals.