In several years of flying helos at low level, inspecting power lines etc (far lower than even our illustrious Chinooks go), landing in fields over horses, in fields next to horses, I've yet to see one properly spooked. Yes, some run around a bit but quickly quieten down, it is my opinion that horses are far more sensible than most riders give them credit for, and also are far more sensitive to the mood of the rider than most give credit for. If the rider becomes frightened (perhaps in anticipation of what the horse hadn't, until then, thought of doing) or angry then this is communicated to the poor horse which then reacts accordingly.
The worst behaved horse I've seen have all had riders on their backs...
Is this the helicopter's fault?
JUst like poultry and sheep slaughter, imho 95% of horse/helo trouble is invented for other motives.