The Keymaster, also known as Vinz Clortho, is the "Terror Dog" who possesses Louis Tully in the first Ghostbusters movie. 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 Keymaster holds one half of the ability to open the door allowing Gozer the Gozerian access to our dimension. The Gatekeeper 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. Vinz Clortho's horns are longer and thinner than Zuul's.
Vinz Clortho had the ability to possess humans and change forms, even while possessing a human.
Vinz first appears as a large horned beast with red eyes known as a Terror Dog. He emerged from Louis Tully's closet during a Louis's party he was holding for his clients. Vinz then, in Terror Dog form, chased Louis out of the building and cornered him at a restaurant where he attacked and possessed Louis' body.
Unlike the more seductive tempered Zuul, Clortho was more slow-witted, confusing the Gatekeeper with regular animals in attempts to identify its partner, and repeating human behaviors and speech to mimic humans in close proximity.
When Walter Peck shut down the Ecto-containment unit, The Keymaster managed to escape and find The Gatekeeper, with whom he shared a passionate kiss at Dana Barrett's apartment. Later, they were finally able to mate at the rooftop of the Shandor Building, transforming into their true demonic Terror Dog forms after they opened the gateway to an alternate dimension through which their master, Gozer entered with the intent of destroying the world.
After Gozer was defeated by the Ghostbusters, Vinz disappears from this dimension and Louis Tully is transformed back into a human being, having no recollection of the events that took place since his and Dana's 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.)