My daughter's horse is fine with flash but flash could certainly spook a horse if it is unfamiliar with it, and of a skittish disposition. If you don't get any better advice here is what I suggest. I wouldn't expose a horse to flash for the first time with a
model on the horse. If the horse gets spooked and panics it could be ugly. I would try it with the horse on a lead rope held by someone familiar with, and trusted by, the horse. I wouldn't point the flash directly at the horse first up. I'd fire off the unit pointing about 90 degrees from the head. If it is comfortable with that then narrow the angle and fire again and repeat until you are pointing at the horse. Only if the horse is remaining comfortable at that point would I put someone on the horse and trial the flash again. The first person on the horse should be an experienced rider familiar with the horse. If that goes OK then put your
model on. Maybe give the horse a treat between the early flashes...piece of bread, chunk of apple something like that, and at the end of the shoot.
From what I've seen flash doesn't seem to bother cows.
Good luck.