The concept of Mega Evolutions were first introduced in thee Sixth Generation of Pokemon, which provided a special evolved form of certain Pokemon. These evolutions bring with them New typing (sometimes), abilities, and Stats. However, this feature isn't available within this game until after a player has finished the main storyline and has defeated Dexio on the Ancient Poni Path, when he upgrades you Z-Ring with a Key Stone. Some features have change through with Mega Evolutions: the Speed and Ability of Mega Evolutions in now taken into consideration the turn the Pokemon Mega Evolves, and Rayquaza cannot Mega Evolve if it knows Dragon Ascent AND is holding a Z-Crystal.
Unfortunately, not all the Mega Evolutions are available within the game at the current moment. Only 21 Pokemon that can Mega Evolve can presently do so in this game - as well as two Pokemon that can undergo Primal Reversion - or after their species can be transferred through Poke Bank. Twenty of these Pokemon can receive their Mega Stone at The Battle Tree, either being given them or purchasing them on their own; Rayquaza can learn Dragon Ascent in Seafolk Village, the process to Mega Evolve it; and Groundon and Kyogre's Orbs can be purchased from the Antique Mall in Hau'oli City to complete Primal Reversions.
Available Mega Evolutions:
Mega Vanusaur
Mega Charizard X
Mega Charizard Y
Mega Blastoise
Mega Alakazam
Mega Slowbro
Mega Gengar
Mega Kangaskhan
Mega Pinsir
Mega Gyarados
Mega Aerodactyl
Mega Scizor
Mega Sableye
Mega Sharpedo
Mega Absol
Mega Glalie
Mega Salamence
Mega Metagross
Mega Rayquaza
Mega Garchomp
Mega Lucario
Available Primal Evolutions
Primal Kyogre
Primal Groudon