We have chickens. LOT'S OF THEM TOO.
They won't let a horse pile stand more than a few seconds on the ground without spreading it around. No pile of manure = no place for the flies to breed. It really works.
Plus we get more eggs than we know what to do with, Fresh chicken and I also haven't seen any rattlesnakes since we got them.
We have 25 acres and the chickens are all free ranging. We have about 60 adult chickens now with 5 horses and a bunch of goats. It's not uncommon to see a rooster out picking on the back of a horse or goat either. They all get along great. We have very little fly problems any more thankfully. We still have some mosquitoes but we have another type of bird for that. Lots of barn swallows. I spray the fence lines and standing water off the property also for the mosquitoes. When we ride the horses off our property I spray them with deep woods off. It's the only thing that works. I haven't seen a single product for horses yet that does work.