The Zilla species. Same powers as the original, can reproduce asexually, lay eggs that fill the entirety of Madison Square Garden in an insanely fast amount of time (the movie never clearly specifies if hours, minutes, months, or days had passed), and can appear and disappear within the blink of an eye and without a sound.
Consider the ecological implications and you realize that this is a potential End of the World scenario as the Zilla species reproduces so rapidly that they'd probably eat the rest the earth's life into extinction within just a couple generations.
The unmade sequel's script actually addresses this. Specifically that not all Zilla's live to adulthood. By the time the survivor is found in the Australian Outback, it has reproduced...though there are only about 20 Zilla's there. The rest died of natural causes such as killed by predators.