Giddy Up!

Now available at Level 20, training is 4 Gold, the mounts themselves are only 1 gold.

Just got "Giddy Up" automatically when hitting level 20 out in the middle of Darkshore with my Worgen hunter. It is possible that hiteSPARKYZ is correct, since Worgen's have the racial Running Wild "mount" and is automatically given to you. I also did not see any drop in my currency, so the training was essentially instant and free. Was usable immediately for me.

There is a quest you can do before getting this achievement, that tells you to go to the riding trainer and learn the riding skill. The quest is called: Learn to Ride in <Zone Name>, and you get it by mail a while after you gain level 20. It should be noted that if you have already learned to ride, then you won't receive the mail and you won't be able to do the quest.

EDIT: The quest has been removed from the game. You will no longer receive a mail telling you to visit your trainer, instead you will be notified upon hitting level 20.