In the 1991 film Godzilla vs. King
Ghidorah, Godzilla's orgin in detail (at least for the "Heisei"
series) was revealed! Godzilla was originally a dinosaur, named
the "Godzillasaurus," who lived on Lagos Island. In
1994, during World War II, a Japanese garrison was attacked by
Americans on Lagos. As the battle continued, Godzillasaurus appeared,
and killed the American forces. An offshore battleship attacked
the Godzillasaurus, and wounded him. Before the Japanese garrison
left, they visited their wounded savior, and thanked him for
saving their lives. In 1954, an H-Bomb test took place on a nearby
island called Bikini. The radiation mutated the Godzillasaurus
into the original Godzilla.
In 1992, people from the 23rd
century came back to the 20th century and asked for help to transport
the Godzillasaurus far from Lagos Island, so Godzilla will not
exist in the future. The Futurians, as the people from the 23rd
century became known as, traveled back to 1944, and teleported
the Godzillasaurus to the Bering Sea, far away from the H-Bomb
test that would take place in the future. Godzilla's creation
though still could not be avoided. The Godzillasaurus, who was
slowly turning into Godzilla, attacked a modern nuclear submarine,
and grew to be 100m tall, twice as big as the original Godzilla.
The Futurians plan to rid Godzilla had failed.