Remember that the horse possibly won't have seen a dog before, so take it easy. Maybe try and train the dog to be very responsive around the horse. I'm not sure how big your dog is, but maybe you could hold the dog in your arms, and obviously don't go straight towards the horse just edge into him with the dog. Also the horse needs to trust you.Otherwise there is no way you'll get the dog near it. Try to strenghthen your bond with him. If there is any possibility that the dog may bit or even snap at the horse muzzle her. You don't want the horse being scared yet again.
Hope I haven't confused you, and that I'm not rambling. Good luck and keep us posted.
Missy

__________________