Dark variant of Mewtwo appears in Pokkén Tournament footage
A dark variant of Mewtwo has been seen in an "Extra Battle" in Pokkén Tournament. When the Extra Battle is unlocked, a cutscene plays which shows the variant Mewtwo Mega Evolve into the variant Mega Mewtwo X.
The Japanese player who discovered it mentioned that they are unsure of the exact unlock conditions, but that they played three single player matches of which they lost the first two before it appeared.
In the cutscene, a black Mewtwo with an orange glowing crystal attached to it drops onto an empty arena. When it lands, purple and orange crystalline structures spread throughout the area. It then Mega Evolves, and the game cuts to a challenger screen. It is controlled by an unnamed Trainer.
No detail of the lore behind this variant is currently known.
Note: The original video is on NicoNico. This is just a YouTube mirror.