Rayquaza was revealed to have the ability to Mega Evolve in Pokemon Omega Ruby and Alpha Sapphire. It has a new ability called Delta Stream which creates a new weather effect that removes weather and makes moves that Flying-type Pokémon are usually weak only deal normal damage to any Flying-type Pokémon. It also has the move Dragon Ascent, exclusive to Rayquaza, that is said to be the most powerful flying-type move. Mega Rayquaza is a Dragon/Flying type, and the reasoning behind it not getting a Primal Evolution and instead a Mega Evolution will be said in the story, and it will mega evolve in a different way from other Pokemon with the capability to Mega Evolve.
What do you think of Mega Rayquaza? I'll post more information as it comes out. Leave a comment below.