The concept of Mega Evolutions was first introduced back in the Sixth Generation of Pokemon. Mega Evolutions are extremely powerful forms that a number of Pokemon that can't normally evolve are able to turn into. Even though Mega Evolutions continue to exist in this game, only the Kanto Pokemon that have the ability to Mega Evolve can do so since only Kanto Pokemon can be found within this game. Only one Mega Evolution is allowed in any given battle, but you must own the Key Stone and the associated Pokemon's Mega Stone in order for said Pokemon to change forms.
Kanto Pokemon Mega Evolutions
Vensaur
Charizard (Mega Charizard X & Mega Charizard Y)
Blastoise
Beedrill
Pidgeot
Alakazam
Slowbro
Gengar
Kangaskhan
Pinsir
Gyarados
Mewtwo (Mega Mewtwo X & Mega Mewtwo Y)
Looking to find all the Mega Stones found in the game? Check out our page on How To Obtain All Mega Stones!