Zilla is a giant mutated iguana originating from a nuclear test in French Polynesia. As the test was done in 1968, the creature (or perhaps species) grew over a period of approximately thirty years. His irradiated genes caused him to achieve a height of over 60 meters (about 200 feet).
He swam from the islands across the Pacific, attacking a Japanese freighter, and then stomped across Panama. From there he traveled up the American Eastern Seaboard, where he sank several American fishing boats. He then stomped through New York, necessitating the evacuation of the entire city. The military battled him extensively, and supposedly killed him in the East River. However, he had laid at least a hundred eggs in Madison Square Garden (an arena in Midtown Manhattan), which hatched and filled the Garden with hundreds of baby Zillas. The military bombed the Garden, killing the creatures. However, Zilla then revealed himself to have survived, and chased the heroes through New York across the Brooklyn Bridge, where he became entangled in the suspension cables. He was then killed by F-18's (although later on, in the sequel TV series, he was resurrected as a cyborg by the Tachyons, this cyborg variant appeared during the monster wars story arc which was very similar to the general plot of Destroy All Monsters and Godzilla: Final Wars). However, one of his offspring survived, and imprinted upon Dr. Niko Tatopolous. From there, he accompanied Tatopolous and his team, H.E.A.T., on their missions against various monsters mutated by nuclear experiments in Godzilla: The Series.
Godzilla: The Series
Zilla appears in the beginning of "New Family - Part 1", the first episode of the TV-series. He later returns in the episodes "Monster Wars, Part 2" and "Monster Wars, Part 3" as Cyber Zilla.
Godzilla: Final Wars
In Godzilla: Final Wars, Zilla is one of the monsters controlled by the Xiliens, and under their control he attacks Sydney, Australia. Zilla is taken away by the Xiliens to give the illusion that they stopped him, and is later unleashed by X to battle Godzilla. Zilla is defeated rather quickly by Godzilla himself, who sends him flying into the Sydney Opera House and finishes him off with his atomic breath.
Zilla's appearance in the film was mainly as a take that to the 1998 film, as he is intentionally rendered using bad CG, and the fight between him and Godzilla lasts 13 seconds, as the shortest in the entire series.
Godzilla: Rulers of Earth
Zilla is a major antagonist in the first two issues of the comic Godzilla: Rulers of Earth, where he comes ashore and attacks the city of Honolulu. Godzilla appears and does battle with Zilla, forcing the monster to retreat.
Zilla later reappears swimming in the waters around the Monster Islands in the thirteenth issue.
In the last issue, Zilla returns to join Godzilla and the other monsters in the battle against the Trilopods. He kills many of them, but he is finally defeated by Magita. After Godzilla managed to destroy it, Zilla follows him into the sea along with the other monsters.