It flies forever through the ozone layer, consuming meteoroids for sustenance. The many meteoroids in its body provide the energy it needs to Mega Evolve.
– Pokémon Alpha Sapphire Pokédex
Mega Rayquaza was introduced in ORAS, and is the focus of the post game Delta Episode.
Instead of getting Primal versions like Groudon and Kyogre, Rayquaza has a Mega evolution. However, it’s different from other mega evolutions, in that instead of requiring it to hold a mega stone, it can mega evolve if it knows the move Dragon Ascent. This means that it can still hold an item (other than a Z-Crystal). Dragon Ascent, despite having “dragon” in the name, is a Flying type move with 120 base power, which lowers the user’s def and sp def by one stage after attacking.
It gets the ability Delta Stream, which is a powercrept version of Air Lock. In addition to causing weather-changing moves and abilities (eg Rain Dance or Drizzle) to fail, Delta Stream makes moves that are super effective against Flying types neutral instead. ie Rock moves are neutral against Mega Rayquaza, Ice moves are only 2x super effective instead of 4x super effective.
Stat-wise, it has the highest base stat total of all Pokemon, tied with the Mega Mewtwos, at 780. It gains stats in all of its stats, mostly in Attack and Sp Attack.