Pokémon evolutions by happiness
This is a list of Pokémon that evolve after becoming friendly. The majority of these Pokémon
baby Pokémon that were added as pre-evolutions in later games.
You can increase a Pokémon's happiness in many ways. The simplest is walking around with the Pokémon in your party, battling and growing them, but not letting them faint. You can also give them a Soothe Bell to hold, which will increase happiness at double the rate. You can also give your Pokémon massages at various places and certain berries raise happiness. For more info, see this PokéBase question: What's the easiest way to raise a Pokémon's happiness?
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.