Pokémon: 10 Weirdest Evolution Methods
8. Shelmet To Accelgor And Karrablast To Escavalier
Ever since generation one, there have been Pokémon that evolve via trade, with Haunter, Kadabra, Graveler, and Machoke paving the way for later generation Pokémon such as Clamperl and Dusclops to follow suit. This method of evolution is not particulary rare, and can be seen happening throughout all seven regions.
What is rare, however, is the relationship between Shelmet and Karrablast, two Pokémon introduced in generation five that evolve via trade. The difference being in this case that they can only evolve into Accelgor and Escavalier respectively when traded for each other.
This is the only example throughout the entire Pokémon world of two creatures sharing this particular bond. According to the Pokédex, this is due to an electrical stimulation the two receive from each other when traded, though how it works and why it is exclusive to just these Pokémon remains a mystery.