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.

It is able to soothe the other members of the super-ancient Pokémon, even in their Primal Forms. Rayquaza is said to have lived for hundreds of millions of years in the ozone layer, where it feeds on water and other particles. Rayquaza is rarely seen by people; even in flight, its appearance is indistinguishable from that of a meteor. It can breathe in outer space. Rayquaza preys on the Minior in the Alola region. Rayquaza also feeds on meteoroids, particularly Mega Stones. Rayquaza can be summoned by a Key Stone. Rayquaza possesses some of the strongest attack power out of any Pokémon; it can easily summon cyclones to overwhelm its opponents. Rayquaza was worshiped by the Draconid people along with the people of Hoenn as the savior of the region. Thus, they built the Sky Pillar to honor it. It is one of the longest-lived Pokémon species.

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 symbol appearing on its forehead. Rayquaza's eyes now contain yellow pupils and 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 tendrils that each extend to a green, fin-like structure; these tendrils have ring patterns that strongly resemble the yellow markings running along the length of Rayquaza's normal form. While in flight, these tendrils give Mega Rayquaza a Δ-shaped 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 tendril 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. Particles stream from the long tendrils that extend from its jutting jaw; these particles can control the density and humidity of the air, allowing Mega Rayquaza to manipulate the weather. Its green hide sparkles with an emerald-like quality.

Mega Rayquaza was said to have been born from humanity's wish for salvation, channeled through the Key Stone of of a Draconid Lorekeeper. Mega Rayquaza is the first recorded Pokémon to achieve Mega Evolution. Rayquaza is at its full power when in the Mega Evolved state. Inside Rayquaza's body lies an organ that contains the same power as a Mega Stone. A Devon Corporation scientist named this organ the mikado organ (Japanese: ミカド器官). By consuming meteoroids as it flies through the stratosphere, the mikado organ will be filled with enough energy to enable Rayquaza's Mega Evolution. Mega Rayquaza has the power to undo the Primal State of Groudon and Kyogre. As seen in the anime Rayquaza can Mega Evolve without a Key Stone. It is capable of destroying meteors bigger than itself as seen with the Grand Meteor Delta.

(Source: Bulbapedia)