A poltergeist in fact.
A primal spirit that has existed on the Earth since prehistoric times
It's mind is that of a dinosaur that is eternally going through the motions of being a dinosaur.
Like the ghost of a soldier who constantly repeats his march across the field of battle upon which he had died.
Godzilla's spirit, not knowing that he is already physically dead, believes he is still alive.
An Earthbound Spirit trapped within his corporeal form in a flash of Thermonuclear fire!

Dinosaurs were the absolute peak of evolutionary life forms on Earth.
Their diversity filled every imaginable ecological nitch until, by whatever means, meteoric or otherwise, doomed them to extinction
. When a spirit leaves the body, it is usually allowed to "step into the light" to merge with its spiritual source.(Heaven or God)
The dinosaurs that were killed by the extinction event, died so quickly that they were deprived of their spiritual transition.
Their spirits were thrust into a dinosaurian PURGATORY where the last living memory they had was a flash of bright light and a huge wave of heat, radiation and pain!

The Godzillasaurus, one of the last remaining living true dinosaurs, was deprived of its "White Light Spiritual Transition Event."
At the time of its death by the intervention of a thermonuclear event!
Another event resulting in bright light, searing heat, radiation and pain.
Godzillasaur's transition was split asunder and all the SPIRITUAL ENERGY of the Dinosaur PURGATORY poured out of the source and into the body of the dinosaur.

Something along the lines of spiritual demonic possesion!
It literaly became the living spirit of ALL the dinosaurs that had died from that explosive extinction event!
It's actions may even be influenced by possesion of HUMAN spirits that died in a similar instant and horrible manner. (Nagasaki and Hiroshima!)
Perhaps explaining its affinity for Japan.

The early speculation that it was a peculiar mix of bipedal carnasaur and armored stegosaur may have been close to right.
It is an amalgam of ALL those dinosaurs and more!
Its Bodyform is a mixed-up concensus of what all dinosaurs aspired to become. Bigger! Stronger! Impervious to all harm!
Because the last, living memory of all those spirits was of light, heat, radiation and pain, they all chose a Bodyform that could not only be immune to these things, but be able to inflict them upon others, in self defense.

Thus came into being the Physical form that we refer to as the Classic ORIGINAL Godzilla !
His body is held together, telekineticly, by the will and self-image of those spirits.
That is why his body does not conform to, or have the weaknesses of biological entities.
He is held together by his thoughts.
He insists upon believing He is still alive.