Mega Rayquaza has been confirmed in the ongoing stream for Omega Ruby & Alpha Sapphire on niconico and in an all-new trailer for the games.
Mega Rayquaza will gain the ability Delta Stream. This Ability protects all Flying Type Pokémon, halving their flying type weaknesses(such as electric, ice, rock). Delta Stream will also cancel the effects of any abilities that introduce weather to the battle, thanks to the new Strong Winds weather condition it introduces. Rain Dance, Sunny Day, Sandstorm, and Hail will fail if Strong Winds is active.
Mega Rayquaza also gains an exclusive new move: Dragon Ascent. The move has one of the highest power ratings of all Flying-type moves. Dragon Ascent is not only powerful, but looks amazing.
Almost all of Rayquaza’s stats receive a boost when the Pokémon Mega Evolves.