The Quantum Courser from the Megadungeon turns into a random mount (from the Past) and some datamining has lead us to a list of possible mount drops, which includes the Infinite Timereaver.
Megadungeon Omni-Mount
The Reins of the Quantum Courser mount was added as an extremely rare drop from the Patch 10.1.5 megadungeon, Dawn of the Infinites.
