Feed the horse a Golden Apple
Feed the horse an Enchanted Golden Apple
Feed the horse a Golden Carrot
Feed the horse Wheat
Feed the horse Hay Bale
Feed the horse Sugar
Feed the horse Apple
Feed the horse Bread
Feed the horse a Dandelion
Feed the horse a Beetroot
Feed the horse a Beetroot Sugar
Feed the horse a Glistering Melon Slice
Feed the horse a Pumpkin Pie
Wait for natural regeneration if the horse is tamed and not taking damage
Use a Splash Potion of Healing on the horse
Use a Lingering Potion of Healing near the horse