The unique Tarecgosa model is used as the mount, much like how players transform into a drake using the Vial of the Sands. Tarecgosa cannot carry another player as a passenger, however. From the screenshots, the mount version looks bigger than most drakes, noting its legendary status. Very, very cool. I like the fact that Blizzard has added in something that will let the legendary staff live on beyond its usefulness, as opposed to just looking nice when you equip it.
Again, big congratulations to Siiz and Premonition. Even though Siiz is a gnome, we won't hold it against him.