Zuul is The Gatekeeper, minion of Gozer. It is a large, horned, dog-like beast with blazing red eyes known as a Terror Dog which first appears inside Dana Barrett's apartment as she comes home with groceries. Zuul appears inside her refrigerator in what appears to be a satanic Temple of some sort, it growls out it's name when Dana notices it, causing her to scream in horror and go to the Ghostbusters for help. Zuul later manifests itself in the real world atop the Shandor Building's rooftop, breaking out of a gothic statue of its own form. Later Zuul possesses Dana as she arrives home to get ready for her date with Peter by having demonic claws pin her to her sofa while it magically slides into the kitchen where Zuul awaits her.
Zuul transforms Dana Barrett into a seductress whom must now find The Keymaster, who has likewise possessed Louis Tully, Dana's nerdy neighbor. Zuul aggressively attempts to seduce Peter Venkman, to which he responds refusing her advances, insisting that he wants to speak to Dana. Zuul (through Dana's body) begins levitating above her bed while growling & roaring monstrously at Peter, who later sedates her.
Zuul later finds her Keymaster. They mate in the demonic shrine atop Dana's building and bring their master, Gozer, into the world before they transform back into the beastly demons which originally broke out of the statues.