MonsterVerse Godzilla's Original Form

After extensive researching and careful consideration, I believe that I may have found the animal that would eventually evolve into what we know as MonsterVerse/Legendary Godzilla... Enter Oradectes sanmiguelensis, a Diadectid reptiliomorph from the Early Permian of what is now Colorado.

Diadectids, like this Diadectes, were among the very first large-bodied, terrestrial herbivores. They existed from (at least) the Late Carboniferous to the Late Permian. MonsterVerse Godzilla is known to have been the (supposedly) sole-surviving member of his species.

Liaoningosaurus paradoxus is a dinosaur whose evolution parallels that of Legendary Godzilla.

Think about it: from a group of large-bodied, herbivorous tetrapods came at least one member that evolved an aquatic lifestyle, including sharp claws and teeth (despite still retaining a beak).

Like Crocodilians, Legendary Godzilla swam from extinction.

And to top it 

Liaoningosaurus paradoxus is a dinosaur whose evolution parallels that of Legendary Godzilla.

Think about it: from a group of large-bodied, herbivorous tetrapods came at least one member that evolved an aquatic lifestyle, including sharp claws and teeth (despite still retaining a beak).