Gyarados 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):
Gyarados, known in Japan as ギャラドス (Gyarados), is a abominable and hostile Water-typePokémon that, despite not being able to fly, is also a Flying-type. Despite not being a Dragon-type, this sea serpent is considered by many a dragon. Surprisingly enough, this atrocious Pokémon is the evolutionary form of the pathetically weak Magikarp, which evolves into Gyarados at Level 20.
Gyarados is a serpentine Pokémon, resembling a Chinese dragon, and not much of a carp anymore. Excluding its head, it consists of fourteen segments. Most of its segments have a yellow underbelly, and yellow spots on each side of each segment. On the first, second, tenth and twelfth segments is a large, spiny dorsal fin. There's a large tail fin attached to the last segment, which appears to be triple-pronged, and have white, serrated membrane between each prong. It has a triple-pronged, dark teal crest on its head, and under its crest are two, menacing eyes placed with red irises and thin pupils. On each side of its head are three blunt spikes, and under these spikes, on each side, is another white fin placed as well as long barbels (Male Gyarados have blue barbels, while female Gyarados have white ones). Finally, Gyarados's face features a huge gaping mouth, with thin yellow lips and four small fangs, as well as a broad snout placed above the upper lip.
In contrast to Magikarp, Gyarados is infamous for its bad temper and destructive nature. It is capable of destroying entire cities when enraged, and only calms down when everything is destroyed. There have been records of this Pokémon's rampages, lasting for months. It is said that this violent nature is shaped because of a structural transformation in Gyarados's brain cells after it has evolved. Even though Gyarados appears rarely in the wild, it usually appears when human wars rise up. It burns down its targets by a powerful Hyper Beam.
When it Mega Evolves, its body becomes bulkier and turns into a darker shade of blue. It now has eleven segments in contrast to fourteen. Its underbelly turns black, rimmed with blood red lines and the spots on each segment become red and bumpy. Its dorsal fins turn yellow, and the ones on the first and second segments become two huge sail-like dorsal fins, placed on both segments. the other two dorsal fins are now on the seventh and ninth segments, and these segments also have a yellow pelvic fin each. its tail fin is also yellow, but smaller too. The crest on its head is also black, and the middle spike has become enormous. its facial fins also become longer and spikier, and its barbels turn white (no matter which gender) and grow as long as Gyarados itself. A fin-like appendage has grown under Mega Gyarados's chin.
Mega Gyarados is even worse tempered than regular Gyarados, and Trainers will have a harder time training Mega Gyarados than any other Mega Evolved Pokémon. In fact, trainers will only battle together with Mega Gyarados when they share deep connections to their Gyarados.
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.
Gyarados is the very first Pokémon which Shiny version didn't have to be caught by random, as a Shiny Gyarados has been rampaging at the Lake of Rage. Therefor, it is the first Shiny Pokémon for many Trainers.
Gyarados can be seen as a counterpart of Milotic. Both are serpentine Pokémon that evolved from a weak fish Pokémon. Both appear in times of conflict (Gyarados to destroy, but Milotic to becalm). Both have a Base Stat total of 540, but their stat distributions are different (Gyarados is a physical attacker, but Milotic isn't).
In addition, it and Milotic are tied for the greatest base stat total increase after evolving, with a difference of 340 in comparison to their basic form.
Despite having the same height as regular Gyarados, Mega Gyarados is the tallest of all Mega Evolved Pokémon.
Gyarados is one of three Pokémon that can be found at level one in the wild. The other two Pokémon to share this distinction are Magikarp and Regigigas.
Gyarados is likely based on a Sea serpent. Magikarp's transformation into Gyarados is based on a Chinese legend about the Dragon Gate: people believed that carp that leapt over the Dragon Gate would turn into dragons. Many waterfalls in China are speculated to be the location of the Dragon Gate
Gyarados may come from 虐殺 gyakusatsu (massacre/slaughter) or 逆境 gyakkyō (hardship/adversity) - the former referring to its violent nature and the latter referring to its time before it evolved. Both words begin with 逆 gyaku, which means "reverse" or "contrary", which refers to it being the complete opposite of Magikarp. It may involve 嵐 arashi (storm) or 争い arasoi (conflict, referencing on how it appeared during wars) as well. Lastly, it may also come from ドス dosu (onomatopoeia for the piercing of flesh, again due to Gyarados's violent nature).
Names in other Languages
From 虐殺 gyakusatsu/逆境 gyakkyō/逆 gyaku, 嵐 arashi/争い arasoi and ドス dosu.