The Ghoul was a ghost that the Ghostbusters captured at a chemical plant.
The Ghoul was causing havoc at Alabaster 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.