Gyrados is my favorite.
What. What do you mean he's not a mythological creature. So your gonna lower him to the status of a POKEMON?
THERE HAS BEEN A GOD-LIKE POKEMON. Don't tell me that I can't consider Gyrados to be an awesome mythological creature.
To be more serious, according to Japanese Mythology, when an Asian Carp jumped over a certain waterfall, it would become a dragon. Gyrados was inspired by this.
Dragons are interesting because their interpretation is surprisingly similar in a lot of different countries. Most asian countries consider them snake-like, while more western countries consider them to be more like a dinosaur (which is quite strange).