Godzilla

 Godzilla is an ancient, mutant Godzillasaurus. His first apperance was in the well known movie Godzilla it is also known as Gojira, which is Godzilla's japanese name.

Origins
Godzilla's origins will depend on the film. Though some films don't have his origin as they are related to each other. Sometimes Godzilla will be a mutant Godzillasaurus, and in other films he came from ancient origin and was awakened by bombings. In the Showa era he is an ancient creature that is feared by many people. While in the Heisei era he is a mutated godzillasaurus, a godzillasaurus is a surviving species of dinosaur in the Heisei era. Though in the Millenium era his origins are not explored much for only in basically one film the original film from 1954's Godzilla skeleton was put inside of MechaGodzilla 3 also known as Kiryu.