Since Pokémon Red/Green were released there have been two more generations of Pokémon, adding some evolutions to Kanto Pokémon that didn’t exist in the original games. How do you get these Pokémon? You simply have to defeat the Elite Four and obtain the National Dex. If you try to evolve a Pokémon before that, it will simply act as if you cancelled the evolution.
Espeon and Umbreon
These two Pokémon are special because they require evolving at a specific time of the day. Pokémon Fire Red/Leaf Green, however, do not keep track of time. This means it is impossible to evolve these two Pokémon in Fire Red and Leaf Green. The solution is to either trade them from Pokémon Ruby/Sapphire or Pokémon Colosseum.