Eevee, the Evolution Pokemon, was a Pokemon released in Generation 1. At the time, it was the only Pokemon that had multiple evolutions (Pokemon like Gloom and Poliwhirl don’t count because their alternate evolutions were released in another generation). For instance, you could get a Vaporeon With a Water Stone. With a Fire Stone, you can get a Flareon, or you can get a Jolteon with a Thunder Stone. In Generation 2, Espeon and Umbreon were introduced. Their evolutions would have been impossible in Generation 1 because there was no day/night cycle. You would get an Espeon if your Eevee gained a level when its happiness was high enough in the day or an Umbreon at night. Generation 3 offered no new Eeveelutions, but Generation 4 introduced Leafeon and Glaceon. These also would have been impossible in previous generations since the rocks necessary to get Eevee to evolve into either one of these were not present. Eevee would evolve into a Leafeon if it leveled up near a Mossy Rock or a Glaceon if it leveled up near an Icy Rock. Generation 5 offered no Eeveelutions either but Generation 6 did. Sylveon, the Intertwining Pokemon, evolves from Eevee when it has two Affection Hearts from Pokemon Amie and knows a Fairy type move (Eevee can be found on Route 10, where it will be at most level 21. The next time it learns a Fairy type move, Charm, is level 29). But should you evolve your Eevee into a Sylveon?
If you don’t have a Fairy type yet, you should. It can be somewhat challenging to get, but it can be worth it. Its ability isn’t the best, but it can catch some of your opponents off guard. Cute Charm makes any Pokemon that uses a contact move against Sylveon have a 30% chance of getting infatuated (if they’re the opposite gender). This fails to work on genderless Pokemon or Pokemon with the ability Oblivious. Your Sylveon will probably be male since only every 1 out of every 8 Eevee is female. Its hidden ability is much more useful but it can’t be obtained without using a Friend Safari. Pixilate is like Refrigerate but for Fairy types.
Fun Fact: Sylveon is the only Pokemon released in Generation 6 that has either a pre-evolution or evolution that was released in a different generation.