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 a free set of either Elven Horse Armor or Steel Horse Armor. Subsequent horse armor may be purchased for 500 .
If a horse is not owned before going to Snak gra-Bura, she will give a free one with armor.
Occasionally, glitched or unfinished notes in certain caves labelled "Horse Armor Plans" can be encountered. They count as quest items and cannot 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.'
Although I had no horse, Snak gra-Bura outfitted her old nag with steel armor and gave it to me free of charge. 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.