Not centaurs, not hippogriffs. Not Nightmares. And, as you said, not a Firesteed. Ooh! You could do Death's pale horse! Or the Four Horsemen! Or an Elvensteed, like the old stories, with the fire-eyed black horse that lures a human onto its back and then runs all night, anywhere and everywhere. Or a Kelpie! The horse with reeds for a mane and tail, that does pretty much the same thing only it drags its victims down to the bottom of its lake.