This item drops from Attumen the Huntsman in Karazhan. It can also be listed on the Black Market Auction House.
Here you can buy Fiery Warhorse for EU or US Servers.
You'll get guide for getting Fiery Warhorse
• You'll be able to get Fiery Warhorse
• After patch 10.2 might will need character transfer for access to BMAH
