The top ranked arena teams of a season one got a GM Prize Token, With it, they can purchase it in a quest and Choose between the Groundmount and the 310% Swift Nether Drake. But now on, you can get the Ground Mount Fiery Warhorse's Reins as a drop from first boss Attumen the Horseman in Karazhan, rare drop.
But you cant get the 310% drake as far as i know, more then become top rank arena team of a season!