It depends which version you are playing.
Red/Blue version: You can find Koffing in the Pokemon Mansion, however he is rare in Blue version.
Yellow Version: He cannot be found in Yellow.
Gold/Crystal/Silver version: You can find him in Mahogany Town or the Burned Tower.
Ruby/Sapphire/Emerald version: You can find him on the Fiery Path, however he is rare in Sapphire version.
FireRed/LeafGreen version: You can find him in the Pokemon Mansion, however he is rare in LeafGreen.
Colosseum: He cannot be found in Colosseum.
XD: He cannot be found in XD.
it is koffing. Koffing i think is the one.
Koga's Pokemon are: Koffing Muk Koffing Weezing
Koffing is in Lilycove City.
Koffing evolves into Wheezing, starting at level 35.
Koffing evolves into Weezing at level 35 in Pokémon FireRed. James uses a Koffing in the anime.
Koffing is #109 in the national pokedex, and it is a Poison type Pokemon.
Koffing evolves at Level 35 in Pokémon Ruby.
koffing
koffing is not real,but you can get it in a Pokemon game.
34-38
you have to trade from firered
Koffing evolves at level at level 35 into Weezing.