The Ghoul was a ghost that the Ghostbusters captured at a chemical plant.
The Ghoul was causing havoc at Alibaster Chemical and led the company to enlist the services of the Ghostbusters. They chased it into a research and development center for chemicals. It ran through some of the experimental chemicals, luring Peter Venkman into shooting at the vats. As a result, a very large chemical spill followed. The Ghostbusters emerged from the spill and captured the Ghoul.
The Ghoul was originally classified as a Class 3 Vaporous Apparition. Egon Spengler had classified the ghost as a Class 3, but it was mysteriously labeled as a weak Class 2 after it was captured. This was later revealed to be because the chemicals that got on the Ghostbusters during the bust had absorbed some of its ectoplasmic energy and that it was developing into a monster. The monster was destroyed after it was formed from the layers of slime growing on the Ghostbusters.