Horu

I'm talking mechanically, not just for flavor. Even though the dex entry for Slowbro in Yellow mentions it'll devolve if the Shellder detaches, the fact that it can't happen in the game means it's just flavor rather than an actual mechanic, considering Slowpoke just evolves into it at a certain level and not when a Shellder bites it in game. Same goes for Magnetron and Dugtrio; they don't become those from 3 of their predecessors combining, just by reaching a certain level.

That'd be an interesting concept, though, rather than literally putting two Pokemon together to fuse them, they just evolve into a different variation if you have certain Pokemon in your party, and it consumes it in the process.

Actually no, it wouldn't need to consume it, just be exposed to it in some way. Like maybe have an item equipped to both, then when the affected Pokemon evolves, it takes on the form of the other wearer. Then you keep the other one and keep raising them as you please.

I mean I understand why pokemon just makes the process instant even though most pokemon would technically grow into their larger evos naturally. That being said, going in order of the dex, the first evo stone we are exposed to is the Thunderstone.