Here’s a long bulky list:
Old: (I think they all return) Mega Abomasnow, Mega Absol, Mega Aerodactyl, Mega Aggron, Mega Alakazam, Mega Altaria, Mega Ampharos, Mega Audino, Mega Banette, Mega Beedrill, Mega Blastoise, Mega Camerupt, Mega Charizard X, Mega Charizard Y, Mega Diancie, Mega Gallade, Mega Garchomp, Mega Gardevoir, Mega Gengar, Mega Glalie, Mega Gyarados, Mega Heracross, Mega Houndoom, Mega Kangaskhan, Mega Lopunny, Mega Lucario, Mega Manectric, Mega Mawile, Mega Medicham, Mega Metagross, Mega Mewtwo X, Mega Mewtwo Y, Mega Pidgeot, Mega Pinsir, Mega Sableye, Mega Salamence, Mega Scizor, Mega Sharpedo, Mega Slowbro, Mega Steelix, Mega Tyranitar, Mega Venusaur
New: Mega Barbaracle, Mega Chandelure, Mega Chesnaught, Mega Clefable, Mega Delphox, Mega Dragalge, Mega Dragonite, Mega Drampa, Mega Eelektross, Mega Emboar, Mega Excadrill, Mega Falinks, Mega Feraligatr, Mega Froslass, Mega Greninja, Mega Hawlucha, Mega Malamar, Mega Meganium, Mega Pyroar, Mega Scolipede, Mega Scrafty, Mega Skarmory, Mega Starmie, Mega Victreebel, Mega Zygarde
DLC (that have been announced)
Mega Raichu X and Y
(https://m.bulbapedia.bulbagarden.net/wiki/Mega_Evolution)
