The secret boss of Pokken Tournament has been revealed. After a 10-year absence, it seems that Shadow Pokemon are relevant again since Shadow Mewtwo is now canon. Its color scheme is a cross between Shadow Lugia and Primal Dialga, and can Mega Evolve to Mega Mewtwo X.
(it appears at 2:02 in the video below)
https://www.youtube.com/watch?v=aJRtBiFD144
Also, it seems to have stolen one of Frieza's attacks:
https://www.youtube.com/watch?v=UkTx2EaDYqI
It also resembles Oni from Street Fighter now.