Years of chain-chugging potions in Dalaran's sewer system have allowed him to grow large, agile, and shadow resistant.
The mount is attached to a letter from Raethan:
We found this creature clawing at an unidentifiable corpse in the back tunnels of the Underbelly, and we were immediately reminded of you. I sincerely hope you find it to your liking.
- Patch 7.0.3 (2016-07-19): Added.