Where two kinds of poison gases meet, two Koffings can fuse into a Weezing over many years.
– Pokemon Red/Blue Pokedex
Weezing is apparently formed with two Koffings (not 3, like Magneton and Dugtrio) bonding together. Unlike Koffing with its happy face though, Weezing’s faces look sad and depressed. Maybe it’s unhappy to have to deal with another gas ball? Its DPPt and BW Pokedex entries actually say that “Though very rare, triplets have been found.” I wonder if that small ball where the two heads are joined is where the 3rd head would be.
According to its Silver Pokedex, “Top-grade perfume is made using its internal poison gases by diluting them to the highest level”, and the Emerald Pokedex further says that it feasts on garbage fumes. What do you know, Weezing turns garbage gas into top grade perfume.
In the anime, James had a Weezing, which he used for most of Kanto. It left Team Rocket to protect some Koffing that were freed from a poacher.
Stat-wise, Weezing has pretty good Defense. Its ability of Levitate makes it so that it’s only weak to Psychic, so it can make a decent wall.