Wowheadnews has a few lovely 3D model previews of a few pets that were datamined from the 4.3 PTR. The previews include the Sand Scarab, the Lunar Lantern (alliance), the Festival Lantern (horde), and last but not least... a "Chromatic Dragon Pet"! [source]
Chromatic Dragon Pet
Information on this curious mini-pet is next to none at the moment. This mysterious companion doesn't even have any distinguishable spell or item IDs yet.
Wowheadnews speculates that it will be the unlock-able guild reward for completing the 4.3 legendary weapon guild achievement, but until Blizzard is willing to offer up more details about the legendary quest chain and possible rewards, we'll just have to wait and see!