#130 - Gyarados
Gyarados is likely based on a sea dragon or sea serpent. The fact that it evolves from Magikarp may also imply that it could be based on the Chinese legend of the dragon gate. In this legend, carps that jump over the dragon gate will become dragons. The dragon gate is said to be a waterfall located somewhere in China. Gyarados also seems to be based upon a Chinese-style dragon.
Further evidence for this theory can be found in Pokemon Snap. In the game, to obtain a photo of a Gyarados, you have to get a Magikarp to swim up a waterfall.
How to get Gyarados in Pokemon Snap
gyakusatsu (massacre/slaughter) OR gyakkyō (hardship/adversity) + dosu (Japanese onomatopoeic word representing the piercing of flesh) OR dos (indicating that Gyarados is second in its evolutionary line)
I don’t know Japanese, this name theory (as do most of the theories I have trouble with) comes straight from Bulbapedia.