Pokémon GO is introducing an old mechanic from the Pokémon franchise called Trade Evolution. Some Pokémon will only evolve if you trade them with another trainer.
The Trade Evolution mechanic is simple. You have the second form of a Pokémon in your collection and ask a friend to trade with you. When you trade, your Pokémon go to your friend. When he gets to them, the Pokémon has a lower candy cost to evolve. If you've already received one of the Pokémon that can evolve through trades in the past, they won't have candy costs to evolve.
From there, your friend makes another trade to send your new Pokémon back to you, and you now have the strongest Pokémon to start fighting inside Pokémon GO.
This method doesn't work for all Pokémon. Here are the ones that benefit:
If you have any of these Pokémon in your collection, contact one of your friends to start trading with them. This mechanic is supposed to help trainers looking to save candy with these Pokémon who want to use it to level them up and make them stronger.
We can expect this list of Pokémon capable of evolving through trade to grow in the future. This is a brand new mechanic for Pokémon GO, but a standard from Pokémon Red and Blue. Because it is a brand new system for Pokémon GO, we can't expect too many PokEmon to have this, but we can expect Niantic developers to slowly introduce. more as time goes by.