In the Japanese films, Catzilla is depicted as a gigantic prehistoric creature, and his first attacks on Japan are linked to the beginning of the Atomic Age. In particular, mutation due to atomic radiation is presented as an explanation for his great size and strange powers. Catzilla's iconic design is composed of a mixture of various species of dinosaurs; specifically, he has the body and overall shape of a Tyrannosaurus, the long arms of an Iguanodon, and the dorsal fins of a Stegosaurus.
Catzilla appeared from the sea, but looks like a terrestrial dinosaur. Through time and as more Catzilla mature, his appearance changed to reflect the concerns of its time. Also his change in form corresponded to a change in character. In his latest incarnation, the dorsal fins are much bigger, sharper and more menacing. The colours also changed from white to purple, to match the skin colour which is now deep green. However; his color is more vivid as they appear very vibrant.