For Pokemon like Kadabra, Machoke, and Haunter to evolve into Alakazam, Machamp, and Gengar, you need to trade them. But you do not need to trade them back in order to evolve them, just once. But they should trade back to you.
Here are a list of Pokemon that need to be traded for them to evolve (whether you need an item or not)
- Poliwhirl -> Politoed (holding a King's Rock)
- Kadabra -> Alakazam (no item needed) (Everstone ineffective, Gen 4 onward)
- Machoke -> Machamp (no item needed)
- Haunter -> Gengar (no item needed)
- Graveler -> Golem (both forms & no item needed)
- Slowpoke -> Slowking (holding a King's Rock)
- Onix -> Steelix (holding a Metal Coat)
- Rhydon -> Rhyperior (holding a Protector)
- Seadra -> Kingdra (holding a Dragon Scale)
- Scyther -> Scizor (holding a Metal Coat)
- Electabuzz -> Electivire (holding a Electirizer)
- Magmar -> Magmortar (holding a Magmarizer)
- Porygon -> Porygon2 (holding a Upgrade)
- Porygon2 -> Porygon-Z (holding a Dubious Disc)
- Feebas -> Milotic (holding a Prism Scale) (not in RSE, DPPt, ORAS)
- Dusclops -> Dusknoir (holding a Reaper Cloth)
- Clamperl -> Huntail (holding a Deep Sea Tooth)
- Clamperl -> Gorebyss (holding a Deep Sea Scale)
- Boldore -> Gigalith (no item needed)
- Gurdurr -> Conkeldurr (no item needed)
- Karrablast -> Escavalier (when traded with Shelmet)
- Shelmet -> Accelgor (when traded with Karrablast)
- Spritzee -> Aromatisse (when holding a Sachet)
- Swirlix -> Slurpuff (when holding a Whipped Dream)
- Phantump -> Trevenant (no item needed)
- Pumpkaboo -> Gourgeist (all sizes) (no item needed)
Reference: Click here to see the Pokemon that evolve when traded.
