Godzilla is the first Kaiju encountered by mankind, in January 2006. It is incredibly large, among the largest of Kaiju.
Godzilla is possibly the last of an ancient species of tylosaur, shaped by millions of years of evolution into a creature suited to an amphibious lifestyle. It does seem to have radioactive properties, though the reasoning behind this is still unclear. Some theorize that it evolved the ability to use nuclear waste as a secondary food source due to high amounts in its natural habitat.