My opinion on the topic
Magikarp and Feebas are like other "duos" in Pokemon. Others include Metapod and Kakuna, or Beautifly and Dustox. They're created as a sort of duo that people would like to use purely for their novelty. They could have changed along the evolutionary line of growth throughout the ages, who knows?
This is just my two cents, it is not necessarily true, but I hope I helped even a little!