As the WoW 17th Anniversary event starts for Asian servers, players have been able to learn more about the new mount, Illidari Doomhawk and other drops from the «new» World Boss Doomwalker in Tanaris.
WoW Anniversary Celebration Guide
Note that information about the World Boss is preliminary, subject to change, and will be updated in the next couple hours and days Information about the Illidari Doomhawk Mount Low droprate — Presumably 1%, but not enough data at the moment to be certain. Read more on wowhead.com