There are 25 Pokémon that can evolve only if you trade them with someone else. You can either trade with a friend, with someone in the game, or over Nintendo Wi-Fi Connection. After you trade, the Pokémon will then evolve while you're still connected to your friend.
Second generation trade evolutions and fourth generation trade evolutions evolve Pokémon that were from previous generations of Pokémon.
Red, Blue and Yellow
In Pokémon Red, Blue and Yellow, the first generation, there were only 4 Pokémon that could evolve by trading. These Pokémon didn't require anything special other than trading. These were:
- Kadabra - Alakazam
- Machoke - Machamp
- Gravler - Golem
- Haunter - Gengar
Gold, Silver and Crystal
In the second generation, they introduced held items. Now certain Pokémon could evolve by trading only if they were holding a certain item. Here are the Pokémon and the held item necessary to evolve.
- Poliwhirl - Politoed (King's Rock)
- Slowpoke - Slowking (King's Rock)
- Onix - Steelix (Metal Coat)
- Scyther - Scizor (Metal Coat)
- Seadra - Kingdra (Dragon Scale)
- Porygon - Porygon2 (Up-Grade)
Ruby, Sapphire and Emerald
In the third generation games, there were only two evolutions that resulted from a trade, and that was a split evolution from Clamperl. You could only get one of these from Clamperl, so choose wisely, or just get another Clamperl and trade it with the other item.
- Clamperl - Huntail (DeepSeaTooth)
- Clamperl - Gorebyss (DeepSeaScale)
Diamond, Pearl and Platinum
In the fourth generation games, they introduced even more held items to trade with. The Pokémon that you can evolve by trading and the held items are:
- Rhydon - Rhyperior (Protector)
- Electabuzz - Electivire (Electirizer)
- Magmar - Magmortar (Magmarizer)
- Porygon2 - Porygon-Z (Dubious Disc)
- Dusclops - Dusknoir (Reaper Cloth)
Black and White
In the fifth generation games, new Pokémon can be evolved by trading. Note that Karrablast and Shelmet only evolve when traded with each other.
- Boldore - Gigalith
- Gurdurr - Conkeldurr
- Karrablast - Escavalier (When traded with Shelmet)
- Shelmet - Accelgor (When traded with Karrablast)
X and Y
In the sixth generation games, four new Pokémon can be evolved by trading.
- Spritzee - Aromatisse (Sachet)
- Swirlix - Slurpuff (Whipped Dream)
- Phantump - Trevenant
- Pumpkaboo - Gourgeist
That's about all there is to trade evolutions! Enjoy trading your Pokémon!