Some of the most ancient vertebrate Pokémon are Fish Pokémon, the group contains some of the more interesting and widespread species. Gyarados (Magikarpus atrocius), or more specifically its Magikarp stage, is probably the most famous example of fish Pokémon, found in many environments (salt water and fresh water alike) and almost every continent. This species also shows a peculiar adaptation, a serpent-like body that allows it not only to move faster in water but also float outside of the water thanks to its new flying typing. This evolutionary quirk can be seen in one other species, Milotic (Feebas tenderis). The two species have been associated with each other by many authors, but modern genetic and morphological analysis have shown that the trait is a case of convergent evolution.

Fish Pokémon also present cases of extreme adaptation, such as the emblematic Wugtrio (Wiggletum tripletus) and Tatsugiri (Tatsugirus falsedrakum), two species that evolved the ability to live outside of the water, and many other peculiar characters.

Inside this group we also find what many have declared to be the “missing link” between Fish Pokémon and Tetrapod Pokémon, Swampert (Mudkipus lungshavens)

A recent new study on one of the fossils the authorities confiscated from Ms. Cara Liss in 2019 and preserved in the Wyndon Natural History Museum, revealed a very ancient species of Fish Pokémon, Plakovish (Plakovium bigbiteus).

This species of Fish Pokémon lived from around 380 and 360 million years ago, during the Devonian period, and was one of the first apex predators of any ecosystem. The fossil shows a peculiar head, reinforced by armor plates composed of dermal bones which made its bite extremely strong. According to some authors, this bite was so strong that it could crush metal, however according to other researchers the methods used in these studies are fallacious and do not represent the actual bite strength.