Fiery Warhorse's Reins

The Fiery Warhorse's Reins summons the Fiery Warhorse Midnight, Attumen the Huntsman's personal steed. The droprate seems to be around 1%.

Source
This item drops from Attumen the Huntsman in Karazhan.

It can also be listed on the Black Market Auction House by Mei Francis, for the opening bid of.

Mount Journal
Was one of the most vicious steeds stabled in Karazhan.

Achievements
Obtaining this mount awards the Feat of Strength Fiery Warhorse's Reins (achievement).