Rayquaza

Rayquaza is a large, green, serpentine Pokémon that also resembles a missile. It has red-tipped, rudder-like wings on its shoulders and down its body, and similarly patterned fins on the tip of its tail. Yellow ring-like symbols run across the length of Rayquaza's body and it has an additional yellow ring on top of its head. Rayquaza has two limbs with three-clawed hands, and two long, flat, horn-like structures on its head, with two shorter horns along its jawline. It has small, yellow eyes with black pupils, prominent pink gums, and two discernible fangs in its upper jaw.

Rayquaza is said to be an equal to both and Kyogre. Befitting its righteous nature, several millenia ago, it halted their clashes and forced them back into slumber, even in their Primal forms, and sealed them away in the Seafloor Cavern, restoring order to nature in the process. For this, it was worshipped by the Draconid people, who refer to it as "Lord Rayquaza" and their "Dragon Lord" in Pokémon Adventures, as the savior of the Hoenn region. Thanks to its weather manipulation powers, it can cancel out weather effects. It has a strong and endless desire to do heroic deeds for the Pokémon world. Rayquaza has resided in the ozone layer for millions of years, where it feeds on water, meteoroids, and other particles. It is rarely seen by people; even in flight, its appearance is indistinguishable from that of a meteor. It is capable of breathing in outer space beyond the ozone layer. It loves Rayquaza Treats made by Snacksworth, who once encountered it in the past. Rayquaza possesses a strong rivalry with the extraterrestrial, Mythical Pokémon, , that repeatedly invades its territory on its way to the Pokémon world, with the two often clashing with each other in space. This is depicted in the movie Destiny Deoxys (M07) and in the games Omega Ruby, Alpha Sapphire, and Masters EX.

Forms
Rayquaza can Mega Evolve into Mega Rayquaza.

As Mega Rayquaza, its body becomes longer and gains more features. Its head becomes longer with the Δ (delta) symbol appearing on its forehead. Rayquaza's eyes now contain yellow pupils with red irises, and black, shichishito-shaped markings trail behind its eyes. Its red lips and teeth become gold, and the gold lines connect to those on its lower horns, which take on a broad, triangular shape. At the outer edges of its lower horns are long, golden, glowing filamens that each extend to a green, fin-like structure; these filaments have ring patterns that strongly resemble the yellow markings running along the length of Rayquaza's normal form. While in flight, these filaments give Mega Rayquaza a triangular profile. Its two upper horns become longer with a gold line running over them in a V shape, and each upper horn has another glowing filament that ends in a circular shape with a spike. Along the length of its body are golden, glowing orbs placed where its ring patterns used to be. The chin, part of the neck, and parts of its serpentine body are changed to the color black. The fins running along its body are now marked with gold lines; the fins on its tail become entirely yellow and substantially longer.

According to the Draconid people, Rayquaza was the first Pokémon to ever Mega Evolve. In its honor, they built the Sky Pillar, where they guarded the rainbow stone that granted Rayquaza the power to Mega Evolve and recorded its many feats on the murals. Unlike other Pokémon, Rayquaza does not have a corresponding Mega Stone to Mega Evolve; instead, it has an internal organ that fills the same role. A Devon Corporation scientist named it the "mikado organ" (Japanese: ミカド器官) and proposed that the combination of its meteoroid diet and own internal energy creates an effect similar to a Mega Stone; this effect, however, is not everlasting, and Rayquaza needs to keep eating meteorites in order to be able to Mega Evolve. The filaments sprouting from Mega Rayquaza's jutting jaw are made of particles that can control the density and humidity of the air, allowing it to manipulate the weather, mainly in the form of strong winds that fill the air and reduce the damage Flying-type Pokémon receive from super effective attacks. Mega Rayquaza is said to have been born from humanity's wish for salvation channeled through a huge meteorite. It is thought that occurrences of Rayquaza Mega Evolving led to the mechanism behind Mega Evolution to be known, which led to the current theory of Mega Evolution.

Mega Rayquaza has enough power to repel the Primal forms of Groudon and Kyogre by sapping up their power with a blazing light and whipping wind. It is also capable of destroying meteors over six miles in diameter. It has been referred to as "Δ (Delta)". There have been sightings of Mega Rayquaza flying gracefully in the sky, resembling a shooting star streaking across the night sky after a meteoroid.

(Source: Bulbapedia)