The latest Nintendo Direct has aired and revealed five new Mega Evolutions for Pokémon Legends: Z-A. The first three are of the original Kalos starters from Pokémon X&Y - Chesnaught, Delphox and Greninja. These are not the First Partner Pokémon
in Legends: Z-A but can be caught during gameplay. Their Mega Stones are obtained from Ranked Battles. Here is the trailer:
Also revealed was Mega Dimension
- a paid DLC rather than being in the base game. The story appears to be based around the mythical Pokémon, Hoopa. During this DLC two new Mega Evolutions will be available - Mega Raichu X and Mega Raichu Y. Images below.