Earlier, a brand new Pokemon was announced. This Pokemon has an appearance similar to Mewtwo, and has been nicknamed 'Mewthree' by many fans; however at the time of writing, an official name has not been released.
The official Pokémon website says that "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... This new Pokémon shares many traits with Mewtwo, including a light-purple body and piercing, powerful eyes. It is suspected that the newly discovered Pokémon has some sort of connection with the Legendary Mewtwo."
The Pokémon Company hasn't revealed anything else yet, so nobody knows whether this Pokémon is a new Mewtwo forme, or a completely new Pokémon with links to Mewtwo. Stay tuned to Nintendo Feed for more information as it is announced!