I've just made a regional variant of Magikarp that thrives in saltwater and evolves into a different Pokémon instead of Gyarados. They're also much more aggressive than regular Magikarp and travel in groups led by a single Akurodos (evolved form) to raid territory (can take out even Wishiwashi School Form with no fear).
I based the evolved form's design off of a Japanese legendary fish/evil god called Akuru. Link:
http://yokai.com/akuru/