There’s plants that cause a horse to be more sensitive to the sun. Most horses don’t eat them on purpose but some do. It made my weanling colt a few years ago burn so horrible and my vet said that was why. I can’t remember what it is called but it is pretty common here in northern Ca. If your horse has access to pasture may want to look up the plants and grass that are out there just to Make sure none are bad in a form for your horse.