Eevee has an unstable genetic code, which allows it to evolve into one of eight different evolutions. The first three of these evolutions, Vaporeon, Jolteon, and Flareon, were introduced in Pokémon Red and Blue.
See also: Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, Blender (software), Eevee (band), List of generation I Pokémon, Pikachu, and List of generation VII Pokémon.