Horse armor cannot be crafted in Minecraft
Find horse armor in loot chests in structures such as dungeons, mineshafts, desert temples, jungle temples, bastions, nether fortresses, strongholds, and ancient cities
Buy horse armor from leatherworker villagers at higher trade levels
Fish for treasure, where horse armor can sometimes be obtained
Use commands in Creative mode to give yourself horse armor
Equip horse armor by right-clicking the horse with the armor in hand or by placing it in the horse inventory while mounted