When Cataclysm releases December 7th, along with all of the new content comes two new races -- one per faction -- for players to level up. The Alliance race is called the Worgen, which has a fairly rich backstory filled with mystery and a bit of suspense. You don't play the feral worgen that you come across in Duskwood, though -- you end up playing a Gilnean who contracts the worgen curse during your nation's evacuation of the capital city, Gilneas.
Aside from the cosmetic racial ability Two Forms -- allowing you to shift from worgen to human and back while outside of combat -- the race gets another unique perk not available to other races. Running Wild is the worgen "mount" which you can opt to use instead of any of the other mounts available in the game.
While this means that the worgen do not offer any new mounts for players, it's a pretty neat tradeoff. It's not like the Mountain O' Mounts achievement will be terribly difficult to complete in Cataclysm, anyway.
[Video via ClassNotSkill @ YouTube]