Check this out!!! You should be able to find lots of these at a grocery store.
Herbal Index - Naturally Equine - herbs for horses and horse healing products
People have been mentioning garlic. It has been proven that is it causes anemia in horses-that is if you feed them too much.Think about it...in the wild will they eat lots of garlic everyday-no. So feed garlic to them max 3 times per week and not too much!
For insect control all I know of is vinegar(it makes your horse smell like vinegar though). To make the smell better you are supposed to add lemon and to make it good for their coat you can add a LITTLE of olive oil.