mount covers

Those aren't covers, though. They're special limited time event mounts that have their own unique level, speed, and so on. Mount covers have never been done before (though clearly mounts were designed with this thought in mind) so we don't really know how it will work. Will they have separate level, and tiredness? Will they have different speeds? There's no telling. Calling it a "cover" suggests that it wouldn't alter the stats in any way, but you never know.