As you catch more Pokémon, you may transfer some of them to Professor Willow to free up your Pokémon storage. You’ll receive Candy in exchange for transferring a Pokémon, but be careful: you can’t get Pokémon back after you’ve transferred them.
To transfer a Pokémon:
  1. From the Map View, tap the Main Menu   button.
  2. Tap the Pokémon    button.
  3. Select a Pokémon from your list.
  4. Tap the Menu    button and select Transfer.
Note: You can’t transfer your active Buddy Pokémon, favorited Pokémon, or Pokémon currently defending Gyms.
To transfer multiple Pokémon at once:
  1. From the Map View, tap the Main Menu  button.
  2. Tap the Pokémon  button.
  3. Locate a Pokémon you’d like to transfer. Tap and hold until the Pokémon is selected.
  4. Tap any other Pokémon you’d like to transfer.
  5. To transfer the selected Pokémon, tap the Transfer button.
Note: To transfer multiple Legendary Pokémon, Mythical Pokémon, you will first need to enable this option in your app settings.

You’re unable to transfer multiple Shiny Pokémon, Lucky Pokémon, or Buddy Pokémon with whom you’ve reached Good Buddy status or above at a time. Additionally, certain Legendary and some Mythical Pokémon (e.g. Mew, Celebi) are not able to be selected for transfer.