Hedorah, also known as the Smog Monster, is a kaiju monster who first appeared in Toho's 1971 film Godzilla vs. Hedorah. The monster was named for Hedoro, the Japanese word for sludge, slime, vomit or chemical ooze.
In Godzilla vs. Hedorah, Hedorah originates from the Dark Gas Nebula in the Orion constellation, journeys to Earth via a passing comet, and lands in Suruga Bay as a monstrous tadpole, increasing in size as it feeds on the pollutants contaminating the water. It proceeds to rampage throughout Japan, killing thousands and feeding on gas emissions and toxic waste, gradually gaining power as it advances from a water stage, to a land stage, and finally a bipedal Perfect Form that it can switch out for a smaller, flying form at any time. Godzilla confronts Hedorah, but discovers that his atomic ray has no effect on the creature. It is later discovered that Hedorah is vulnerable to temperatures high enough to dehydrate it, so the JSDF constructs a pair of gigantic electrodes on Mount Fuji. Hedorah fights Godzilla there, and is subsequently killed when Godzilla uses his atomic breath to power the electrodes, which cripple Hedorah and allow Godzilla to incinerate its remains.
Hedorah briefly reappears in Godzilla: Final Wars, where it is under the control of the Xiliens. It is swiftly destroyed alongside Ebirah in Tokyo Bay by Godzilla. A 2021 short film created for the 50th Anniversary of the character would give the Final Wars incarnations of Hedorah and Godzilla a rematch amid an oil refinery in the daytime.
Hedorah also appears in Godzilla: Monster Apocalypse, the prequel novel to Godzilla: Planet of the Monsters.