There are lots of neat ground mounts that were released before flying was possible but since they're flagged as non-flying, you can't use them as an appearence mount with your flying mount. For example I have a mount that's a flying carpet, but I can't use it's appearence and fly around, even though flying carpets can now fly. Maybe the ground mounts that are of a flying nature could be flagged as "possible to use as appearence with a flying mount". Such as clouds, flying carpets, and disks. While normal horses, and other obviously grounded mounts would remain unusable for in-air appearences.
Kram337 wrote: While I agree you should be able to use clouds , older carpets etc as appearance , devs have stated that these older items don't have all the 'states' included that would let them 'tip , turn etc' when flying , they won't/can't spend the development time to add them , so it's unlikely we'll ever be able to do this.