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.
A shiny Eevee was obtainable in Beta 4.0 as a New Year reward.
The only three offensive moves Eevee can learn by TM (Shadow Ball, Iron Tail and Dig) are types that Eevee cannot evolve into, meaning none of its evolutions can receive STAB from them.
Evolving into Nucleon will override all of Eevee's other level up-based evolutions, meaning that you must remove any Nuclear Pokémon in your team before you can evolve into Espeon, Umbreon, Glaceon, Leafeon, or Sylveon. Alternatively, you can evolve into those Pokémon before level 20.
According to Motofumi Fujiwara, a key person at Game Freak, Eevee was based on a vague childhood experience he had, He described seeing an "indefinable creature" and that Eevee was based on a drawing of all that he could remember of it.
Eevee comes from E V, the first two letters in the word evolution.