Blizzard just posted this image of a new hippogryph mount and refers to it as a "reward" which means we can probably rule out it being a purchasable pet store item. There are absolutely no hints on what it will be rewarded from, but I've got a few ideas.
World of Warcraft's 9th anniversary is on November 23. Could we be looking at a reward for logging in during the anniversary? The last time we saw a reward like this was for the 5th anniversary when everyone received an Onyxian Whelpling pet. The only thing working against this theory is the last 3 anniversaries have just been feat of strength rewards. The more likely possibility is a new Recruit-A-Friend or Scroll of Resurrection mount. It could even be an in-game reward, but for what?
Let the speculation begin.