Pokémon evolutions by Friendship
This is a list of Pokémon that evolve after becoming friendly. The majority of these Pokémon are
baby Pokémon that were added as pre-evolutions in later games.
A Pokémon's Friendship can be increased in many ways. The simplest is walking around with the Pokémon in your party, battling and growing them, but not letting them faint. Massages and haircuts increase Friendship by a larger amount. Giving them a Soothe Bell to hold increases the effect of the other methods. See more in the glossary or this PokéBase question.
This information applies to any games that feature both Pokémon from the chain, so for example in Generation 1 the evolution chain with Pichu doesn't exist.