Actually, nothing. I did a LOT of research (1 google search) into equestrian photography and the experts all concurred (on 2 forums) that horses are good with flash photography.
The shoot was amazing, the hair and makeup girls were amazing, and the horse, Riley, settled into having his picture taken after a few minor spaz outs. He even smiled in the first photo...
My lovely girlfriend was again modelling and I thought I'd take this opportunity to embarrass her a little since she made me promise I wouldn't put the pic up on facebook.