Rayquaza did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Rayquaza (Japanese: レックウザ Rayquaza) is a dual-type Dragon/Flying Legendary Pokémon.
While it is not known to evolve into or from any other Pokémon, it can Mega Evolve into Mega Rayquaza, but only if it knows the move Dragon Ascent.
Rayquaza is the version mascot of Pokémon Emerald, appearing on the boxart of the game. It serves to end the conflict between Kyogre and Groudon when Team Magma's leader Maxie and Team Aqua's leader Archie awakened them.
Along with Kyogre and Groudon, Rayquaza is a part of the weather trio, serving as its trio master. In Pokémon Omega Ruby and Alpha Sapphire, it plays an important role during the Delta Episode and is required to be caught there.
Rayquaza is a large, green, serpentine dragon. It has red-tipped, rudder-like appendages on its body, one pair behind its shoulders and two quartets below its torso and its waist. Ryquaza's tail features similar appendages on its tip. 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 weather trio. Rayquaza can live 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.
As Mega Rayquaza, its body becomes longer and gains more features. Its head becomes longer with the Delta symbol appearing on its forehead. Its eyes now contain yellow pupils and red irises, and black, tree-like 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. 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.
Inside Rayquaza's body lies an organ that contains the same power of a Mega Stone. A Devon Corporation scientist named this organ the mikado organ. By consuming meteoroids as it flies through the stratosphere, the mikado organ will be filled with enough energy to allow Rayquaza to Mega Evolve.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
By level up
20% chance of making the target flinch.
Prepares the first turn and strikes the next turn.
Lowers the target's Speed stat two stages.
Raises the Speed stat on the user's team for 3 turns.
30% chance of making the target flinch.
20% chance of lowering the target's Defense stat one stage.
Sleeps for 2 turns, restoring HP and status problems.
No added effect.
Raises the user's Attack and Speed stats one stage.
10% chance of raising all of the user's stats one stage.
Lowers the user's Defense and Special Defense stats one stage.
No added effect.
No added effect.
User must recharge the next turn.
Repeats for 2-3 turns then confuses user.
Rayquaza has many differences from the other members of its trio:
It is the only Pokémon with a base stat total of 680 that does not share the specific stat numbers with another Pokémon.
It is the only member without a Primal form, instead having a Mega Evolution.
It is the only member not to be directly sought after by a specific crime syndicate.
It is the only member that cannot learn an OHKO move.
It is the only member that cannot learn its signature move by level-up, as it can learn it by move tutor.
It is the only member that didn't appear in the episodes Gaining Groudon and The Scuffle of Legends.
However, evidence says that it will come if Groudon and Kyogre are battling; in Pokémon Emerald, the player must get Rayquaza to stop the battle of Groudon and Kyogre in Sootopolis manually. This may mean that Rayquaza can only calm the two down but may not know when and where they are fighting.
Mega Rayquaza has the highest Attack and Special Attack stats and the highest base stat total of all Dragon-type and Flying-type Pokémon.
Mega Rayquaza is the tallest Dragon-type Pokémon and Flying-type Pokémon.
Mega Rayquaza is also the tallest Mega Evolved Pokémon.
Mega Rayquaza is the tallest Legendary Pokémon.
Mega Rayquaza is the heaviest Flying-type Pokémon.
In games prior to Pokémon Omega Ruby and Alpha Sapphire, Rayquaza has a catch rate of 3.
Rayquaza's catch rate was raised to 45 in ORAS, likely due to it being a required capture during the storyline.
Rayquaza is the only legendary Pokémon that can be caught at a higher level than the Elite Four's Pokémon before defeating the Elite Four. This only happens in Pokémon Emerald.
Rayquaza was the most voted Pokémon of all pre-Generation V Pokémon in the Korean Pokémon Global Link promotion poll.
Rayquaza was also the most voted on the 15th anniversary poll, which was for the version mascots of the core series Pokémon games, making it Japan's favorite version mascot.
Rayquaza is misspelled as "Rayquayza" in Pokémon Team Turbo in every instance.
Even though Rayquaza uses Dig in Super Smash Bros. Brawl, it has no way of learning the move in any Pokémon game.
Rayquaza is the only Pokémon capable of Mega Evolving that does not require a Mega Stone to do so in the games.
Rayquaza is also the only trio master capable of Mega Evolution.
Mega Rayquaza is tied with both of Mewtwo's Mega Evolutions for the highest base stat total of all Pokémon, with 780.
This makes it the only Pokémon with a base stat total above 720 that doesn't require a specific held item.
Rayquaza's battle theme in Pokémon Omega Ruby and Alpha Sapphire is the only battle theme that can play while the player is capable of walking in the overworld; there is a brief period of time between the cutscene in which Zinnia summons it and the battle itself where the player must approach Rayquaza themselves.
Rayquaza is most likely based on the Hebrew legends of Ziz, the unconquerable primal master of the sky. Its counterparts were Behemoth and Leviathan. Rayquaza's appearance also draws inspiration from other creatures of myth such as the Asian dragon and lindworm. Alternatively, Rayquaza matches the description of the Aztec god Quetzalcoatl, who was depicted as a flying serpent, and created the boundary between the earth (including the oceans) and the sky. Its trait of being capable of inhabiting the skies, and living there while subsisting on nothing but dew and particles for centuries, also bears aspects of the legendary Chinese xian hermit sages and East Asian dragons.
Rayquaza most likely refers to Raqiya, the Hebrew word for "firmament". It also may be a combination of ray and quasar. Alternatively, it may be a combination of 烈 retsu (violent or furious), 空 kū (sky), and 座 za (seat or location), which can be taken to mean "one that sits in the ferocious heavens".
Rayquaza's mikado organ refers to 帝 mikado (emperor), another name for the Emperor of Japan or 天皇 tennō in Japanese, which can be translated as either "heavenly sovereign" or "king of the heaven/sky".
Rayquaza from the Boxart of Pokémon Emerald Version.