I discovered a horse floating one block above the ground. It was floating above a regular grass plant (minecraft:grass). Next to it was its two foals. Upon attempting to ride the floating horse, the horse stopped floating and fell to the ground. There is not a single command block or function/datapack that could be giving a horse the NoGravity NBT tag, repetitively teleporting a horse to one specific block, or placing a barrier block below it. I did not interact with the horse in any way before I noticed it floating. I did, however, see it not floating when I loaded the chunk for the first time, but upon wandering around more and coming back, it was floating.

I have been around many horses in the snapshot and have never noticed something like this. Perhaps it is from the new collision mechanics, which is very likely, for I saw the horse's foals near it and sometimes pushing it.

Recreating
I don't know how to recreate it. It simply happened, and I assume it had something to do with chunk loading. I apologize if this should not be reported because it is too hard to reproduce.