By CaptainPotatoes (Via kysel)
Wait... what? D:
In the Pokémon Smash trailer for next week's episode, it also showed a battle where Mewtwo Mega Evolved while promising a shocking revelation about Mega Evolution, but they covered up the result of the evolution, with the host Shokotan seeming shocked.
This also goes hand-in-hand with the Nordic "Tree Of Life" rumor related to Yveltal and Xerneas. Maybe all Mega Pokémon are based on Nordic Gods and that is why there are a limited number of them.