"Warmer weather means that the bugs are out, and the one pest that our horses seem to hate the most are flies. On the worst days, there may be dozens buzzing around just one horse while out in the pasture. Luckily there are solutions --- the most common being a fly mask, which will protect your horse's face (and particularly the eyes) from these little critters. Fly masks are also great for keeping dust out of eyes or for protecting the face if you have some wooded areas of your pasture."
https://www.agdaily.com/livestock/4-best-fly-masks-horses