With mounts with stats coming into play I believe that you should be able to upgrade your school mount. Not only to add a stat onto it but when you upgrade it, it should transform. Say, when you upgrade the death whirlwind it turns into a Death Strider. This would be a horse covered in armor. However the horse would be completely made of the black particles from the original whirlwind kind of giving it a smokey, ethereal kinda look.
first off I like the idea of them upgrading into new mounts but still keeping the original look or feel to it that would be great. I definitely like the idea of adding stats but it would probably be like the jewels you put on pets