All multi-headed Pokémon or Pokémon that evolve when more of that specific Pokémon congregate now require you to amass that exact number and meet specific qualifications. For instance, if you want Magneton, you have to have two other Magnemite in your party when one reaches a certain level. Same with Honedge into Doublade.

In order to make a Metang, you must combine two Beldum. To make a Metagross, you have to combine two Metang. Want a Dodrio? Grab three Doduo and use some hellish scientific device to splice them together.