Answer:
Eevee, the evolution pokémon, has many forms of evolution here are the current evolutions, the generation each evolution originated in and how to get each one. You cannot obtain evolutions for later generations in previous games. This means it would be impossible to get a leafeon in Pokémon Ruby for example.
1st generation:
vaporeon - use a Water Stone
flareon - use a Fire Stone
jolteon - use a Thunder Stone
2nd generation:
umbreon - max out your eevee's happiness and level it up during nighttime
espeon - max out your eevee's happiness and level it up during daytime
4th generation:
leafeon - have your eevee level up near the mossy rock in Eterna Forest
glaceon - have your eevee level up near the icy rock on route to Snowpoint City