These are two new companion pets added in 4.2, one for Alliance and one for Horde.
Here is how to get the Balloon for Alliance. Most likely Horde has the same system (ask in trade where the quest starts).
You can get the balloon by doing a simple quest that you can pick near the Cathedral in Stormwind, turn “track quests” and you will see it if you stand in front of the Cathedral.
Here is the exact location from where you can get the quest entitled “Blown Away”. It is taken from “Vin”, a draenei female child.
What is the quest about?
The quest is about gathering 5 balloons scattered trough Stormwind.
Tips for gathering the balloons:
Try to find the balloons by flying with the mount, they glow from a far distance. Also it is recommended to roam the areas far from Cathedral as most of the players will just search there.
Here is an example of how a gathered balloon looks like.
And the Alliance Balloon after completing the quest