I have horses next door and cows in the bee yard and no problems at all as long as I keep the bees fenced off so the cow's can't knock over the hives scratching themselves on them. My cows have learned not to stand in the flight path. My bees are far more reactive to my mostly black boarder collie and have often chased and stung him when he gets in the bee yard though they leave my white lab alone. Often on horses the insects that look like bees trying to sting them are in reality bot flies trying to lay eggs on the horse's coat. The eggs are tiny cream colored grains attached to the ends of hairs usually found around a horses fore arms and flanks. The horse will ingest the eggs while scratching itself with it's teeth and the larvae then develop in the horse's digestive system where they can do great harm and in untreated animals actually cause death from malnutrition. Most equine de-wormer products are made to remedy this pest. Horses will react violently to the assault of this fly and run and buck to avoid them. Many people will see the horses acting wildly and notice the fly but assume it is a bee. Look for the eggs as evidence.