In the anime, Ghidorah is the deity worshipped by the Exif under the titles of "Wings of Death", "Golden Demise", and "God of Destruction". He is summoned to Earth by Metphies and his cult in order to destroy Godzilla and devour Earth just as they fed him other planets they visited and converted to the Exif faith.

Because of these tremendous powers, King Ghidorah often requires a combined effort from several parties to bring him down. Only through a union with Mothra or humans can Godzilla defeat this mighty dragon.

As others have mentioned, the aliens numbered everything. Ghidorah was the first, coming to their planet after Earth's monsters drove him off. Because everything is numbered here. The monster, is zero.

At some point long before the events of the film, Venus was home to a vast and heavily-developed ecosystem, complete with an advanced intelligent race of its own, the Venusians. King Ghidorah appeared from space and attacked the planet, eventually wiping out all life.