The Horse Armor Pack was the first downloadable content released for The Elder Scrolls IV: Oblivion, priced at $1.99 (£1.16) on the PC and 200 points on the Xbox 360. It provides steel and elven armor for your horse.
Once the Hero of Kvatch receives the note, simply speak to Snak gra-Bura at the Chestnut Handy Stables outside the Imperial City. She will give them a free set of either Elven Horse Armor or Steel Horse Armor. Subsequent horse armor may be purchased for 500 .
If they do not own a horse before going to Snak gra-Bura, she will give them a free one with armor.
Occasionally, they will come across glitched or unfinished notes in certain caves labelled "Horse Armor Plans". They count as quest items and can not be dropped, although they are not used in any quest and become permanently stuck in the inventory. When reading the document, it says "I'm a set of plans. Go to Chestnut Handy Stables."
I've discovered a notice from Snak gra-Bura of the Chestnut Handy stables. She is offering to outfit a horse with armor at no charge to anyone who presents her with a note like the one that I've found. I should bring my horse to her and take advantage of this offer.
Snak gra-Bura armored my horse for free. From now on, she will outfit any legally owned horse that I bring to her with Steel or Elven armor for 500 gold.'
- Horse Armor at the Official Website – Detailed information about the Horse Armor mod.