Fiery Warhorse's Reins

The Fiery Warhorse's Reins summon a Fiery Warhorse epic mount akin to Midnight, Attumen the Huntsman's personal steed. The droprate seems to be around 1%.

Source
This item drops from Attumen the Huntsman in Karazhan.

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