According to Nintendo - "The mysteries of Pokémon X and Pokémon Y continue to grow with the unveiling of a Pokémon that is strangely familiar! It looks a lot like the powerful Legendary Pokémon Mewtwo, but not quite the same…"
So apparently this newly discovered Pokémon is somehow connected to Mewtwo, but it isn't strictly a new form. I'm betting in X and Y we find out the connection. Could it be Mewthree? This means every friend that told you their dad's friend's brother worked for Nintendo was RIGHT!
Just kidding, but maybe Mewtwo was messing around with cloning itself to create a child type thing... who knows?
This image depicts a new Pokémon, with the Japanese name, Ferion. It is a Light/Flying dual-type Pokémon which could be a new beginning of the game Flying type evolutionary line or possibly a legendary? Who knows...
We also know more about how Light types will work:
Offensively, Light types will be Super Effective against Dark, Fighting, and Steel types. They will not be very effective against Rock, Psychic, or other Flying types
Defensively, Light types will be weak against Ghost, Poison, and Dark types and resistant to Fighting and Light.
Having another type will move the series forward and will really change gameplay. How could you not be excited for this?!
Besides the introduction of a new type, Pokémon X and Y will have these new gameplay improvements and additions:
After you beat the Elite Four you gain access to an island. You can then go back into the world and recruit AI trainers or friends (like Join Avenue) to create a town, and ultimately your own gym.
You can gain abilities by leveling up, like moves. You then choose which ability you want to keep.
IVs and EVs will be shown in game on the Pokémon's stats screen and there will be a tutorial section for them.
You receive two Exp. Shares.
There will be 6 vs. 6 battles.
The level cap can be increased for certain Pokémon. To do this you must go to the new Pokémon Friendship Center and officially befriend your Pokémon. After this, that Pokémon will be able to reach level 120. This Pokémon will also follow you on the field. You can change which Pokemon you are partners with by going back to the Pokémon Friendship Center at the cost of lowering it back to level 100.
All Pokémon will learn a special move at the increased level cap.
All ledges on the field will now be slides.
Every fifth random encounter will be a Zubat you can't run from that only uses Confuse Ray and always goes first.