Showing off a new evolution method in my Pokémon Red hack, Pokémon Prototype!
For these fusions, a certain Pokémon must gain a level (however, it doesn't matter what level it's at) while certain other Pokémon are currently in your party. Then all the Pokémon involved will be removed from your party and will be replaced with the fusion.
For example, if you have a Slowpoke that gains a level while there is a Shellder present in your party, both Pokémon will be removed and replaced with a Slowbro. And so on for three Diglett, three Magnemite, and two Koffing.