It is used to protect your horse from taking damage. There are three types of horse armors.
+ Iron Horse Armor
+ Golden Horse Armor
+ Diamond Horse Armor
Horse armor could only be found in generated chests particularly those of dungeons(dungeon chests, blacksmith chests, Nether Fortress chests, and Abandoned Mineshafts) and also by trading with a villager.
BUT thanks to this mod now you can get all 3 horse armors just in seconds!!!
Unlike most other tools Horse armor does not break over time. and horse armor cannot be enchanted(Unlike regular armors).
If a horse dies while it wearing the armor, the armor will drop as an item into your inventory.