The Gatekeeper, also known as Zuul, is the "Terror Dog" who eventually possesses Dana Barrett in the first Ghostbusters movie. It is a demigod that was worshipped in 6000 BC by the Hittites, Mesopotamians, and Sumerians as a minion of Gozer, the shape-shifting god of destruction. Both the Gatekeeper and the Keymaster have the ability to possess human hosts and then transform into their native "Terror Dog" form when necessary.
The Gatekeeper holds one half of the ability to open the door allowing Gozer the Gozerian access to our dimension. The Keymaster holds the other half of this power. When both the Gatekeeper and the Keymaster are allowed to use their powers together, they alone can open the door for Gozer. One distinguishing feature between the "Terror Dog" versions of Vinz Clortho and Zuul are their horns. Zuul has shorter horns than her counterpart, Vinz Clortho.
Zuul first appears as a large horned beast with red eyes known as a Terror Dog. It appeared in Dana Barrett's refrigerator within Gozer's Temple as she was back from getting groceries. Zuul reappeared behind Dana's kitchen door later when she had just arrived and was about to get ready for a date with Peter Venkman. This time, Zuul successfully trapped her and possessed Dana's body. While possessing Dana's body, Zuul was able to speak, but could not access Dana's memories or knowledge. She took on the identity of The Gatekeeper and immediately sought out Vinz Clortho, another minion of Gozer who had possessed the body of Louis Tully, Dana's neighbor who had a not so secret crush on her. Unlike the more slow-witted Vinz, Zuul seemed more even-tempered and hypersexual. She patiently awaited the Keymaster in Dana Barrett's apartment, first encountering Peter Venkman who
arrived for his date with Dana. Mistaking him for Vinz Clortho, Zuul immediately began to aggressively seduce him. Becoming extremely frustrated at Peter rebuffing her advances, Zuul (in Dana's body) growled like an otherworldly demon while levitating four feet off her bed. Peter sedated her before leaving the apartment to head back to the Ghostbusters headquarters. Zuul later awoke when the Ghostbusters firehouse exploded and blew out the living room windows in her apartment with another supernatural roar as she watched spirits light up the New York City skyline. The demolished walls helped the Keymaster find the apartment and the two possessed tenants shared a passionate kiss before walking toward the Temple of Gozer through a hidden staircase now visible behind where Dana's kitchen door stood before. Later, Zuul and Vinz were finally able to mate at the rooftop of the Shandor Building, transforming afterwards into their true demonic Terror Dog forms while simultaneously opening the gateway to an alternate dimension through which their master, Gozer entered with the intent of destroying the world. Upon completion of their transformation, the Terror Dogs take their places guarding the doorway to Gozer's Temple.
After Gozer was defeated by the Ghostbusters, Zuul disappears from this dimension and Dana Barrett is transformed back into a human being, having no recollection of the events that took place since her and Louis' demonic possession (in a deleted scene, Louis suggests remembering having had sex with Dana Barrett while they were both under possession, to which Dana, unknowingly, believes untrue.)