A capfull of cider vinegar in there drinking water also works great
Vinegar diluted in water works nicely. You can also put some diced garlic in their mash / other food, it repells insects.
My mom home makes her fly spray for all of our horses and it works like a charm with the ingredients of cooking/veg oil and dish soap and pertherine and water and vinegar and mix it
I use tea tree oil like 10 drops half vinegar and half water and if his coats dry. A tablespoon of oil. Any oil cooking/veg. Works like a charm
As many essential oils as you want, veggie oil, olive oil, dish soap, vinegar or any mix of these
Listerine the mouth wash , vinigar , baby oil , and extra coconut oil or and ripe of cheap oil for your hair .