Volcanion is the first ever dual fire and water-type Pokemon, and was first leaked two years ago.
Almost two years ago, a Pokemon X and Y hacker discovered three secret, unobtainable Pokemon for the game. Diancie and Hoopa were officially revealed by Nintendo in short order, but the third, the unique dual fire and water-type Volcanion, remained a mystery. Until today, that is, with Nintendo "officially" revealing the Pokemon.
"This Pokemon uses fire and water to create steam within its body, expelling explosive shock waves and scalding steam from the arms on its back. Volcanion's power is said to be so great it can destroy an entire mountain, gouging out the earth and changing the shape of the land!"
You can read more about Volcanion on his official page, or check out the trailer above. Nintendo has not yet revealed how players will be able to get their hands on him, but now that he has been officially revealed we can probably expect some sort of distribution event in the near future.