Shadow Mewtwo was previously confirmed as a boss character for Pokkén Tournament, but Mewtwo had not previously been confirmed as a playable character.
Garchomp will be able to Mega Evolve into Mega Garchomp as its Resonance Burst. Mewtwo has not been confirmed to be able to Mega Evolve, but Shadow Mewtwo is known to Mega Evolve into Shadow Mega Mewtwo X in the game.
These new fighters will be available in the game when it launches on Wii U in Japan on March 18, 2016. It will be released on Wii U internationally in Northern Hemisphere spring 2016.
There will be an announcement of a new fighter for Pokkén Tournament on January 15, 2016. This fighter is known to have the character 「ラ」 ra in its Japanese name, so it is not any of these newly announced fighters.