Reins of the Deathcharger

Reins of the Deathcharger is a special mount for death knights that can be ridden while in combat.

Source
This item can be looted at any time from the Deathcharger Hitching Post in Acherus: The Ebon Hold over the Broken Isles after completing the Dark Horsemen order hall research.

Trivia

 * This mount is initially identical in appearance to the Acherus Deathcharger. After completing the order hall campaign, however, its appearance is chosen at random from the mounts of the new Four Horsemen: Nazgrim's Charger (green), Trollbane's Charger (red), Whitemane's Charger (white), or Mograine's Charger (blue).