I grew up riding horses around the Middle Wallop area, The horses didn't take any notice of the helicopter activity, because as mentioned they seemed to know it was on its way somewhere else and not chasing them. They can get used to anything with the right training, take police horses for example. But you give them something that makes them feel threatened and they can bolt and that can kill themselves and their rider.
When I was dairy farming in a different part of the country the cows just dozed and chewed their cud when low level RAF aircraft went over. But one day a hot air balloon went over, put on it's hot air or whatever it is they do right over the herd...the cows went completely nuts, fences broken, all over the road, car crashes, injuries, lost milk yield, aborted calves.... Probably because it was so sudden, they didn't hear it "Approaching" and weren't used to it. Believe me it takes a lot to spook a dairy cow!
If livestock weren't spooked by low level aircraft then the mustering folks here in Aus in their R22s wouldn't get much work done!