Rayquaza

Rayquaza is a Dragon/Flying-type Legendary Pokémon who made its debut in the Generation III games, Pokémon Ruby and Sapphire Versions. It is the mascot of Pokémon Emerald Version. Rayquaza's first Super Smash Bros.-related appearance is in Super Smash Bros. Brawl, in which it is the second boss of The Subspace Emissary.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Rayquaza is fought as a boss in the level, The Lake. In it, Rayquaza suddenly emerges from a lake after hearing Diddy Kong, who had been looking at the nearly destroyed Arwing nearby. It attacks the Arwing with Dragon Pulse and snatches Diddy in its claws. Diddy screams for help, whereas Fox McCloud, who manages to escape the damaged Arwing, leaps from the Arwing and breaks Diddy free of Rayquaza's grasp using Fox Illusion. Rayquaza, furious, ends up using Dragon Pulse on Fox, who deflects it with Reflector, flinging it back at Rayquaza, who falls back into the lake.

Fox and Diddy engage in a fight with Rayquaza, resulting in the second main boss fight of the Subspace Emissary. Some of its attacks involve burrowing into the ground and then bursting upwards. It also flies with great speed horizontally over the stage and sometimes drops a Thunder attack. The player can beat it by attacking it around the head. When Rayquaza uses Dragon Pulse, the player can use Fox's Reflector to counter it.

Unlike most enemies, Rayquaza does not appear to have any connections with the Subspace Army, although it is later fought again in The Great Maze. Rayquaza can also be fought in the unlockable mode, Boss Battles.

Super Smash Bros. for Wii U
Rayquaza reappears in Super Smash Bros. for Wii U as a stage hazard for the Kalos Pokémon League stage. It sometimes appear at the Dragon-type chamber, where it damages anyone that it comes into contact with. The game also has a trophy of Rayquaza, which can be purchased at the Trophy Shop.

Super Smash Bros. Ultimate
Rayquaza returns in Super Smash Bros. Ultimate, retaining its status as a stage hazard for the Kalos Pokémon League. It is also featured as a spirit.

Trivia

 * Unlike other bosses of The Subspace Emissary, Rayquaza never interacts with characters from its own series.