Minecraft wiki entry for HorseBase, associated text:  ArmorItem: The armor item worn by this horse. May not exist.
Tags common to all items[show]
 Bred: 1 or 0 (true/false) – Unknown. Remains 0 after breeding. If true, causes it to stay near other horses with this flag set.
 EatingHaystack: 1 or 0 (true/false) – true if the horse is grazing.
 Owner: The UUID of the player that tamed the horse, stored as four ints. Has no effect on behavior. Does not exist if there is no owner.
 SaddleItem: The saddle item worn by this horse. May not exist. (Non-player mobs cannot control a tamed horse if it has no saddle)
Tags common to all items[show]
 Tame: 1 or 0 (true/false) – true if the horse is tamed.
 Temper: Ranges from 0 to 100; increases with feeding. Higher values make a horse easier to tame.