Most horses don't gaze up into the sky that I know of, but the flash would have to be so repetitive for it to do some damage.

I tried it for my garden this year and looks like the bird are not even coming close. I put some see down around my garden to see if the little robbers would come out and try and get it and for the most part.. no.

Nice little trick the part is trying to keep it looking nice. I just used 3 and hung them in a straight line with some 20 pound fishing line. I used an old rake. And that seems to look pretty clean.