Well, you could say you open up a quick portal to your masters staples in the nevernever, so the horse can come to you and assist you. A simple maneuver spell should be good enough, if you invoke for effect. On the other hand, it might even be a -1 worldwalkeresque power to do this properly, but that would be up to you.
I do see some problems with the "horse without stats" thing, because sooner or later, you might want to trample someone with your horse.
Another way I could see it work would be with an "Echo of the Beast". The Echo would let the character roll survival at +1 to move around, as long as he spends one exchange opening his mind for the horses spirit, or something like that (a discipline roll against a number depending on the disturbances around him). That way, the horse would only help him in spirit, without actually being there, and the mechanical effect would be the same.