MrGM has reported that the new Reins of the Quantum Courser mount drops from the Patch 10.1.5 megadungeon, Dawn of the Infinite. When used, this mount transforms into another mount from the past and allows you to learn it!
Reins of the Quantum Courser
The Reins of the Quantum Courser mount was added in the last Patch 10.1.5 PTR.
According to MrGM on Twitter, this drops from the final boss of the Megadungeon, Dawn of the Infinite.
