Three Eevee evolutions were available in Pokémon Red and Blue: Vaporeon, Jolteon, and Flareon. Each evolution represents a different elemental type and is obtained by using a specific evolution stone.
Eevee itself is a Normal-type Pokémon known as the Evolution Pokémon. Its unstable genetic code allows it to evolve into different forms, which helped make it one of the most recognizable Pokémon from the first generation. In Red and Blue, the Water Stone produces Vaporeon, the Thunder Stone produces Jolteon, and the Fire Stone produces Flareon.
The number of Eevee evolutions grew in later generations. Espeon and Umbreon arrived in Gold and Silver, followed by Leafeon and Glaceon in Diamond and Pearl, and Sylveon in X and Y. Because Eevee now has eight recognized evolutions in the main series, modern players sometimes mistakenly apply that later total to the original games.