Note that the vanilla horse armour and Middle-earth horse armour are now listed as separate items, even though they may look and act the same (e.g. diamond horse armour). You can not put vanilla horse armour on a Middle-earth horse or vice versa. The horse armour of different factions can be crafted on that faction's (ie. Morgul horse armour is made on a Morgul Crafting Table). Some horses, namely those ridden by NPCs, may naturally spawn wearing horse armour. Examples of this include horses of the Rohirrim, Swan Knights, Gondor Soldiers, and more.