Even in Generation I, there is various ways of evolving a Pokemon. One of these methods is via various evolution stones introduced in the generation itself, which are the Fire Stone, Water Stone, Thunder Stone, Leaf Stone and Moon Stone.
And the Pokemon that is heavily influenced by these stones is Eevee. In Generation I, it evolves into Flareon via the Fire Stone, Vaporeon via the Water Stone and Jolteon via the Thunder Stone. In Generation IV, when Leafeon is introduced, Eevee can evolve into it via the Leaf Stone.
However, till date, there is no evolution for Eevee via the Moon Stone.
Correction
Um actually … Eevee does not evolve into Leafon via Leaf Stone in Generation IV.
That method is only introduced in Generation VIII.
Prior to that, you would need to level up Eevee near a Moss Rock.