Shelmet and Karrablast only evolve when both traders trade that Pokémon. For example, if you trade Shelmet for a Graveler, only Graveler will evolve, not Shelmet. The same is true for Karrablast. Both Shelmet andKarrablast must be traded through the same trade for them to evolve. They are the first Pokémon to do this.
Shelmet's appearance resembles a knight's helmet, with the visor open.
It also resembles a nautilus.
Shelmet's name appears to be a portmanteau of "shell" and "helmet".
Despite having the metal "shell", it is not Steel-type.
It is shown, when traded with a Karrablast, it takes Shelmet's helmet, deshelling it and causing it to evolve into Accelgor, the Deshelled Pokémon. Karrablast then evolves into Escavalier and wears Shelmet's armor.