When attempting to feed horses, not undead ones, there is no icon to allow the player to feed their horse to regenerate health. There is only the ride or mount button, and only when holding the applicable and expensive food item to breed the horse will a feed option appear. Neither does the option appear when riding the horse.

(Classic and Pocket touch settings)

There is no way to feed horses wheat or sugar to regain hearts, only golden carrots or golden/enchanted apples can be used.