Eevee was the only Pokémon with branched evolutions in Generation I.
In Pokémon Red and Blue, Eevee could evolve into Vaporeon, Jolteon, or Flareon. Each evolution used a different elemental stone, allowing one species to develop into Water-, Electric-, or Fire-type forms.
This branching design distinguished Eevee from the ordinary linear evolution families of the first generation. Eevee itself is a Normal-type Pokémon and is described as having an unstable genetic structure, which explains its unusual ability to adapt into multiple forms.
Later generations expanded the family with Espeon and Umbreon in Gold and Silver, then Leafeon, Glaceon, and Sylveon in later games. The term “Eeveelution” began as a fan expression and later became an official label. Despite Eevee’s many forms, the original games offered only the first three.