Little Valley Homesteading

If your horse is allergic to fly bites, try 1/4 cup of apple cider
vinegar on their grain once a day. This raises the blood acid level
just enough to bother the flies, but it is completely healthy for the
horse. It takes about one week to start seeing the effects. Several
old timers have told me that they always put vinegar in their horse's
drinking water during the summer to repel flies and mosquitoes.

There is  golf course near my home. And the golfers uses  Bounce
 fabric softener sheets to repel insects. Next time you go riding tie one
 to your Horse's headstall and stick one in your back pocket. It works
 better than ANY fly spray I've ever tried. The flies won't even fly around you.