Barbara- I had a similar reaction with a little girl with a strong startle reflex. She was frightened by the horse sneezing. I told her father that she would not be able to continue HPOT if we were not able to manage her fear. We did two things that were very successful. We positioned her backwards to remove the visual stimuli of the head and neck (in my case where the sneeze came from). We also placed a towel over the horse (neck and rump) for a few sessions to decrease the visual response from the patient. I hope my experience may be helpful to you and your patient.
I saw this little guy yesterday and used a different horse and all went fine. The child grew scared very briefly after a small horse sneeze but then I was able to quickly redirect him. I do believe that other horse shook a lot to see if he could get a reaction. I like your idea of facing backwards to take the focus off and create some distance from the horse's sneezing head. i am not sure how the towel helped- did it serve mainly to distract or actually block the view of the horse's head . I have to admit a photo is worth a thousand words- which is why I keep taking them!!!