Nucleon when leveling up to level 20 with a Nuclear-type Pokémon in the player's party.
An Eevee has an unstable genetic makeup that suddenly mutates due to its environment. Radiation from various stones causes this Pokémon to evolve.
Eevee is a small, fennec fox-like creature with bushy, brown fur. Its muzzle is very cat-like, with a small, black, triangular nose. It has a fluffy cream-colored mane around its neck and a short, bushy, fox-like tail with a cream tip. Eevee has round, deep-brown eyes, long, rabbit-like ears, and pink paw pads on its little feet. Its paws are small with three toes and no visible claws.
Eevee, as well as its evolutions, possesses traits from different animals. Eevee shares most traits with the fennec fox. However, it also shares traits with dogs, cats and rabbits. Eevee is said to have an irregularly-shaped genetic structure, enabling it to evolve into multiple Pokémon.