Mega Evolution is a mechanic in the Pokémon series, introduced in Pokémon X and Y. It is a temporary mega evolution that can be initiated in battle if the Pokémon is holding the Mega Stone for their species, however, only select Pokémon can undergo Mega Evolution. It can be triggered in battle before attacking, and it allows the Pokémon to attack after undergoing Mega Evolution. Only one Pokémon per team can be in its Mega Evolution, and after the battle is over any Pokémon in their mega evolution return to their normal form.
List of Mega Evolutions
Super Smash Bros.
Lucario, Mewtwo, and Charizard all have a Final Smash transforms them into their mega evolved form. Lucario's Mega Evolution makes him more powerful, Charizard's Mega Evolution allows him to use Fire Blast temperately and Dragon Rush and Mewtwo's Mega Evolution does Psystrike. Lucario and Charizard can also perform the Undeath Glitch. In Super Smash Bros. Ultimate, Mewtwo and Lucario still had Mega Evolution, but Charizard returned to his Pokémon Trainer team.