Zilla was originally a joyless and deeply unpopular reinvention of Godzilla for an American film audience in the 1990s. Toho, the company that created and still owns the character of Godzilla, renamed this version Zilla "because they took the god [the awesomeness and dignity] out of Godzilla".
Godzilla: Final Wars (2004)
In Toho's film Godzilla: Final Wars, Zilla was sent to attack Sydney, Australia, by its Xillien masters who teleported it back to their mothership when they revealed themselves to make it look like they had killed the beast along with the other creatures they had deployed all across the world.
When Godzilla was freed from his icy tomb in the Arctic, the EDF (Earth Defense Force) skyship Gotengo lead the King of the Monsters to Sydney where he found Zilla waiting for him. Zilla attacked Godzilla on sight, dodging Godzilla's atomic breath, but when Zilla tried to overpower him, Godzilla tossed him into the Sydney Opera House and obliterated the creature with his atomic breath, much to the displeasure of the new Xillien Controller.
Godzilla: Rulers of Earth (2013-2015)
In IDW's comic Godzilla: Rulers of the Earth, the rogue kaiju known as Zilla was first encountered by a submarine whose crew mistook it for Godzilla, upon making landfall in Honolulu, Hawaii, Zilla went on a terrible rampage and clashed with the CKR forces stationed there until Godzilla arrived. The two monsters battled each other but the fight ultimately came down to a draw, which lead to both monsters retreating into the Pacific so their injuries could heal.
Powers and Abilities
Godzilla: Final Wars
Aside from the enhanced speed and agility, this version of Zilla also featured an acidic fire breath originating from its stomach and when in contact with air ignited.
Godzilla: Rulers of Earth
This second version of Zilla had enhanced speed, agility, but lacked the acidic fire breath of the version from Godzilla: Final Wars. It was also incredibly strong, so strong in fact that it was able to face Godzilla on equal ground and end their fight in a draw.