Vaporeon

This POKAEMON has the ability to freely control water. VAPOREON underwent a spontaneous mutation and grew fins and gills that allow them to live underwater. They have the ability to freely control water. Its body's cellular structure is similar to the molecular composition of water. Vaporeon is an Eevee who has adapted to life in the water or has evolved by a Water Stone – its fur has been replaced by smooth, shiny skin, its fluffy mane with a webbed frill, and its puffy tail with a long, sleek dolphin-like one, and a cat like appearance. People who have seen just the tail have apparently been known to mistake Vaporeon for a mermaid. Vaporeon is more than just an aquatic-adapted Pokémon, though. it can actually control water to a degree that few other Water-types can match, can predict the approach of rain, and can even dissolve its own body into water in order to move unseen*

You take a stroll by a Lake and find a wild Vaporeon, you catch it with your Pokeball and take it in as your new pet buddy