Slimer's Dream Ghost is a giant pink dragon-like entity Slimer made up for a dream where he saved the Ghostbusters.
While feeling neglected by the Ghostbusters, Slimer dreamed about being a Ghostbuster. The Dream Ghost chased the unarmed Ghostbusters down an alley. When things looked bleak for the guys, Slimer leaped down from a rooftop to save the day. Clad his own jumpsuit and armed with a Proton Pack, Slimer vaporized the Dream Ghost and earned his place on the team.