Zilla

Zilla is the main antagonist in the 1998 film Godzilla. 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. Its irradiated genes caused it to achieve a height of over sixty meters (about two-hundred feet).

Godzilla (1998)
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, slaying the infant Zilla's. 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 apeared 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 Tatopolis. From there, he accompanied Tatopolis and his team, H.E.A.T., on their missions against various monsters mutated by nuclear experiments in Godzilla: The Series. See Cyber Zilla for the final defeat of Zilla.

Godzilla: Final Wars
In Godzilla: Final Wars, Zilla is one of the monsters controlled by the Xilians, and under their control he attacks Sydney, Australia. The towering reptile topples down buildings, chomps on vehicles, incinerates a crowd with his fire breath, and even eats a couple of civilians as he brings thunder down under! He is defeated rather quickly by Godzilla himself, who sends him flying into the Sydney Opera House and finishes him off with his atomic breath. The battle was only 13 seconds long. Critics say that this was meant to show that the original Godzilla was "better". This incarnation of the reptilian doppleganger had some of highly explosive fire breath. This Zilla appeared to be the same as Zilla but it is very different in color (light gray). So it can be concluded that it was only of the same species but not as strong as Zilla or the stronger Zilla Jr.