I've bred horses and worked closely with them all my flying life. Horses, including highly strung thoroughbreds, are NOT unduly "spooked" by helicopters. I've overflown and landed next to literally hundreds without incedent.
Most ridden horses "spook" because their riders are nervous!
This is a pointless restriction to a most neccesary part of the military flying training regime brought about because the current leadership within the military and MoD is "spooked" by a vociferous pressure group.