Horses are notorious flower munchers, there isn't anything that will stop them if the flowers are just out within reach. In fact my daughter's horse went up 4 steps to the sundeck to get the petunias in urns. I occassionally let my horses out in the yard when it's been raining hard & the lawns are like hayfields (when we built the house, I seeded everything, pastures, lawns with dryland pasture mix) to mow it down a bit before I can cut it with the lawnmower. I tarp over all the flowers otherwise they will eat them down to nubs. Once I forgot about a hanging basket by the carport, it was eaten bare. One thing I can recommend is planting marigolds, they won't eat them, rest are fair game.