Here's what I'd be OK with:
1. Certain mobs with the ability to shoot down flying mounts from the ground. Very few of these and not in all zones. Maybe over enemy (PvP) cities and villages. You would not necessarily die from impact. Perhaps instead of the mount simply poofing, you lose control of the mount and you spiral toward the ground at a slower pace before falling off and taking just a little damage.
2. Flying mobs with the ability to do the same thing. Again, not in every zone, but in some areas. Lore would certainly allow for it.