Marrowfang

Marrowfang, Bonefang's progeny, was respected by Amarth the Harvester for her insatiable appetite, even if it disrupted any experiments that the sadistic commander wanted to run.

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding (ground)
Expert Riding (flying)

Source:

Nalthor the Rimebinder in The Necrotic Wake (Mythic Dungeon).
Reins of the Quantum Courser (chance)

Notes:

Marrowfang has a small chance of dropping from Nalthor the Rimebinder the last boss of The Necrotic Wake dungeon, but on Mythic (and Mythic+) difficulty only.