After running the game with this mod enabled, you'll get a log entry saying that you've found a note from Snak gra-Bura, the proprietor of Chestnut Handy Stables near the Imperial City. It says that you're welcome there to receive your free horse armor. Go there and talk to Snak. If you don't own a horse, you can convince or threaten her into giving you one. In that case, you'll get an Old Nag equipped with a steel armor.
From now on, you can equip any horse with an armor: a steel one or an elfish one. Their functionality is exactly the same (they double the horsey's health), so you should only think about which one you like better.
HINT: You can't buy an armor for a stolen horse, you have to legally own one.