Smoldering Ember Wyrm

Source
This item drops from Nightbane in Return to Karazhan.

Mount Journal
Arcanagos sought to turn Medivh away from experiments into the arcane, but the influence of Sargeras had corrupted the wizard too deeply, and the great dragon paid for the transgression with eternal, burning agony.

Drop chance
The mount has a 20% chance to drop for every player in the group eligible for loot, up to a 100% drop chance if all five players are not loot locked. It will only drop for players who do not already have it.