Getting up close to a Koffing will give you a chance to observe, through its thin skin, the toxic gases swirling inside. It blows up at the slightest stimulation.
(From Bulbapedia)Koffing is a spherical Pokémon filled with toxic gases. It has vacant eyes and a wide mouth that usually has two pointed teeth in the upper jaw. Below its face is a cream-colored skull-and-crossbones marking. Several geyser-like protrusions, which usually release a mustard-colored gas, cover this Pokémon’s body. Its purple skin is very thin, and over inflation can cause it to explode.
Koffing creates gases within its body by mixing toxins with garbage, and produces more gas in higher temperatures. These gases are malodorous and cause sniffles, coughs, and teary eyes to those around it. The gases are also lighter than air, allowing Koffing to hover. When agitated, it creates a more toxic gas and expels it through the holes in its body.
(From Bulbapedia) Considering its looks, its ability to float, and its tendency to explode, it may have been based upon a floating naval mine or living meteorite. It is also based on smog and other forms of air pollution. Koffing may also be based on the imagined fear that heavy pollution, caused by the leakage and/or improper disposal of toxic and/or radioactive waste, may result in the creation of new and undesirable life forms. The crossbone skull under its face is a reference to the toxic hazard symbol.
(From Bulbapedia)Koffing is a corruption of coughing.