I'm nursing an "injury" right now. Stupidest thing. Last week I was riding and was apparently bitten by a horse fly.
This is a rare occurrence - I usually get "got" about once a year - horse flies generally don't care for the taste of omnivore blood. And today I was in the doctor's...the bite got infected and now I have a seven day course of antibiotics to look forward to.
So. Flies and horses. Horses attract biting flies and insects including mosquitos. Horse flies are in the family Tabanidae and are biting flies. The females feed on blood to get extra protein for reproduction (as with most biting insects, only the ladies are dangerous).
And yes, horses are very much bothered by flies, sometimes to the point of accidentally tossing their rider. Fly repellents of various sorts have been used for generations. Horses will kick, buck and bite to get rid of flies. Flies also carry diseases - swamp fever, west nile virus, etc. So, applying fly repellent to horse and rider is part of the routine.
Fly masks are also sometimes used on horses, both for turnout and riding. These masks are mesh and cover the horse's eyes. The "ear covers" you might see on competition horses are also for fly protection, as are the fringes that drop down from them or from a browband - aimed at keeping flies out of the horse's eyes. It's for the comfort of the horse - and the protection of the rider.
And, as I'm being unpleasantly reminded right now, you don't want to get bitten.