He was created as a response to the atomic bombings of Japan in 1945... Radiation killed hundreds of thousands of people and mutated flora and fauna... That's why Godzilla was filmed as a kind of result and counterweight to the use of atomic weapons and nuclear energy in general.
So, in folklore, Godzilla can be considered a mutant, like some kind of bloodsucker in S.T.A.L.K.E.R... He has no relation to dragons, except for his reptilian appearance. And his breath is not fiery, but a special thermal breath – of radioactive nature...