Reins of the Onyx Netherwing Drake

Source
This mount is a quest reward from the quest Onyxien the Onyx Netherwing Drake. It can also be bought from Drake Dealer Hurlunk on Netherwing Ledge in Shadowmoon Valley for. You must be with the faction Netherwing to obtain the quest and the right to purchase the mount.

Mount Journal
After promising to fly you to the ends of Outland, the netherwing drake was eventually drawn to the blue skies of Azeroth as well.