In the very first film, he is often described as an atomic monster that rampages across Tokyo and kills everyone and destroys everything in sight of him.
However, I have two new theories where he was born. The first one is Godzilla is, perhaps, an ancient dragon that terrorized the world going back to the medieval ages. Kingdoms feared him, often saying that if they had done something wrong in humanity, the Gods would send Godzilla to destroy it (when in fact he was just a roaming dragon). Entire armies would reject to fight him, as he stood 400 ft in height making for an impossible victory. His unusual fire breath that resembled the color blue reached a temperature of 1,200 degrees melting anything in its wake. Kingdoms would have a chance, however, at escaping because his roar was often heard more than twenty-five miles away and people had a chance to flee. No modern records indicate such a creature existed because anybody who encountered him or saw him, it would often be too late to tell the story. In 1954 when an atomic explosion detonated too close to his resting place, he was at large once again.
My second theory is Godzilla is a never-before seen specimen of dinosaur buried beneath the ocean floor. Godzilla is possibly more than 66 million years old and originated somewhere in western North America. He was so large that his size compared to that of a Amphicoelias sauropod dinosaur at more than 200 ft in length from head to tail. Godzilla was able to crawl through underground caves (if they were large enough) and so if he was able to crawl, from head to tail would measure almost 500 ft long (as his tail is often never counted with his height as he often stands like a human). This would make him the single largest dinosaur specimen ever to have lived.