A horse can be frightened by a piece of paper, a camera flash, a dog farting, and a passing helicopter. ONLY the helicopter will evoke a response from the horse's owner. Perceived noise nuisance.
I have spent thousands of hours at low level (level with the wire) on power line inspections, and have scared the snot out of lots of horses, because they were hiding behind a line of trees that I overflew. Whenever I saw horses ahead of me, I pulled away from the line, went around the nags at a suitable distance, and rejoined the line. But many times, a bunch of hoses would suddenly start running - straight at me. Horses is such stoopid people.