It's not as though they are Azerothian horses that got corrupted... they're from another
planet which Warlocks summon and enslave them from.
So fel <insert anything other than horses here> makes no sense, really.
Does it not? we know next to nothing about the planet they are summoned from. If they have a species that looks almost exactly the same as a horse, would it be too far fetched to believe they have species that look almost identical to wolves and raptors? I don't think so.