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 change 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.