Right now we have a cool looking blue Hearthsteed mount to ride around on after you've won three games against other random players in Hearthstone. A lot of people really like the mount, and rightly so, it's pretty badass.
But more people are asking for the red version now, a version that has only been datamined. Players incorrectly assumed that because both a blue and red version were released in the game files that one would be an Alliance mount and the other a Horde mount. This spread like wild and has entered into the collective thought process of the WoW community.
In reality the mount is just not being used yet, per Cory Stockton's tweet above. It'll probably be used at some other point -- a lot of sources are pointing at Diablo III's upcoming expansion. So just wait and see. It'll be here eventually, it's too cool not to be.