Pokémon C is an upcomming game, developed by NewbuyGames. It's a Directors Cut of Pokémon A and B, which are currently unreleased.
Pokémon C, like Pokémon A and B, will (probabbly) be released as a Gameboy Color game. It's a Spin-Off of Pokémon Z, the upcoming Directors Cut of Pokémon X and Y. Pokémon C has got all Pokémon, including the Pokémon from X and Y. The story will be the same as the story of Pokémon Z. The only thing what will change are the Graphics, which will be the same as Pokemon Red and Blue.
NewbuyGames: "We are not sure when Pokémon C will come out, but it will be after the Release of Pokémon Z. Pokémon A and B will be based on Pokémon C so A and B will come out a couple of years after C."