In order to Mega Evolve, most Pokémon must be holding their appropriate Mega Stone and the Trainer must have a Key Stone. Only Rayquaza does not need a Mega Stone to Mega Evolve; instead, it must know the move Dragon Ascent. There are 46 species of Pokémon that are capable of Mega Evolution, and there are 48 different Mega Evolutions. SHINY EDITION: https://www.youtube.com/watch?v=td6h545Uxus