I’m just curious if there is a reason you can’t put the horse where you want it if it is trained to stay in one spot and look where you want? I assume the horse will be groomed and look marvelous so you won’t want to mess up the mane and forelock with a halter, but a lead rope draped over the neck to bring it into position and then quickly removed would be easiest - you at least give the photographer one less thing to worry about when working with animals.