Pokémon Go's Buddy Feature: Earn Extra Candy by Walking

Different Pokémon Earn Candy at Different Rates, with Legendary Pokémon Requiring 20km of Walking

Pokémon Go's Buddy feature allows players to earn extra Candy by walking around with a selected Pokémon companion.
Different Pokémon earn Candy at different rates, with some earning Candy for every 1km walked, others for every 3km, and others for every 5km.
Legendary Pokémon require 20km of walking before they can earn Candy.
Players can change their Buddy Pokémon by tapping their Trainer icon, scrolling to the ‘Me’ tab, and pressing the ‘Buddy’ icon.
There doesn’t seem to be a limit to the amount of Candy a player can earn with their Buddy in a day.