There's something about this species. Normally, it's grouped with Trubbish & Grimer lines. However, those two are clearly artificial accidents, whether it be direct Pokedex statements or implications.

There's nothing to imply that Koffing & Weezing are an artificial species. In fact, in a lot of games, they're native to volcanoes (RSE Fiery Path, DPPt Stark Mountain) or at least near it (Cinnabar Island, known for being destroyed by a volcano). What I'm implying here is that they were a natural species that lived in volcanoes and were only drawn out by pollution, and unlike Grimer & Muk who are dying out due to pollution being cleaned, Koffing & Weezing can just return to volcanoes and eat all the volcano things.

Obviously their Galarian counterparts are far removed from the whole volcano thing, seeing how they've developed a typing that's not very effective against Fire, which might be a sign that that population has lived far from volcanoes for far too long.