Where two kinds of poison gases meet, 2 KOFFINGs can fuse into a WEEZING over many years.
It lives and grows by absorbing dust, germs and poison gases that are contained in toxic waste and garbage.
If one of the twin KOFFING inflates, the other one deflates. It con stantly mixes its poisonous gases.
Top-grade perfume is made using its internal poison gases by diluting them to the high est level.
When it inhales poisonous gases from garbage, its body expands, and its insides smell much worse.
— Pokédex entries (via PokeAPI)