Eevee was the first Pokémon in the series with a branching evolution, but is it ever explained why it can transform into so many other Pokémon? In Pokémon Red & Blue, Eevee could turn into one of three Pokémon by using Evolution Stones, which meant that players who wanted to complete the Pokédex had to trade with another player, as they only received a single Eevee in one run.
Eevee could evolve into Jolteon, Flareon, or Vaporeaon using Evolution Stones in Pokémon Red & Blue. The new Friendship and day and night mechanics added in Pokémon Gold & Silver were tied to Eevee's ability to evolve into Espeon or Umbreon. The introduction of location-based evolutions in Pokémon Diamond & Pearl allowed Eevee to evolve into Glaceon or Leafeon, though this mechanic was later replaced with Evolution Stones in Pokémon Sword & Shield. The most recent Eeveelution was Sylveon from Pokémon X & Y, which was added alongside the new Fairy-type Pokémon that were introduced in Gen 6. Eevee needed several levels of Affection and to level up while knowing a Fairy-type move, in order to evolve into Sylveon.
The only Eevee that cannot evolve are the one from the Let's Go games and the ones with the ability to evolve into their Gigantamax form in Pokémon Sword & Shield. Outside of these specific examples, Eevee has the most evolutions of any Pokémon in the world revealed so far. What is it about Eevee that gives it this incredible potential for evolution?
Pokémon's Eevee Is An Evolution Machine Due To DNA
The reason Eevee can evolve into so many different types of Pokémon can be found in the numerous Pokédex entries it has received throughout the series. According to the Pokédex, Eevee has an irregular genetic makeup, with DNA that can be altered by outside stimuli. In the case of Pokémon Red & Blue, this referred to the radiation produced by Evolution Stones, which was also effective on other Pokémon. Later Pokédex entries clarify that Eevee's DNA allows it to survive in a number of different environments, thus explaining its many Eeveelutions.
The ability to influence DNA isn't exclusive to Eevee, as the DNA Splicers from the Unova region are used to give Kyurem new forms. Mewtwo from Pokémon Red & Blue is said to be the result of DNA splicing experiments, while Deoxys is based on and named after DNA. All of these factors, combined with how Fossil Pokémon are revived in the series suggest that the creators of Pokémon were fans of Jurassic Park, which could explain why so many Pokémon like Eevee have alterable DNA.