I've had a couple of instances of this lately - one for a hero, the second for an enemy - is there any rule to say a character can only have one mount attached?
Case 1 - Glorfindel was riding Windfola (restricted), and I wanted to attach Asfaloth to him (unrestricted).
Case 2 - Uruk Chieftain (in Race across Harad) - had a Mordor Warg attached (limit 1 per enemy) and I drew a Racing Warg (also limit 1 per enemy, but I am assuming the limit is for the card of the same name, as neither card text specifies otherwise).
In both cases, I had the target double-mounted, as I couldn't see a rule not to do so.