A lot of the monsters are simply blow up versions of existing animals, this being a list of them.
Mammals
Birds
Dinosaurs
Tyranosaurus Rex
Based on the Tyranosaurs, and not an accurate depiction. An actual tyranosaur would be unable to stand tall as the anime version does and would have a more slender head, as well as be smaller.
Diplodocus
Based on the Diplodocus, and lackings its herbivorism. While the actual Diplodocus did have sharp teeth, this was for grazing. Likewise, it had a long neck, but it was rather still and unable to be lifted very high, unlike the snake neck seen in the anime. They lived in groups and would probably die when walking into the sort of cave that anime Diplodocus lived in, due to being to heavy and stiff to move there.
Emerald Dragon
Based on the Nodosauridae, a dinosaur family closely related to the Ankylosaurs, but distinct in lacking a bony tail, and having spikes pertruding from their back. Actual Nodosauridae were much smaller, lacked fangs and would not be as agile.
Reptiles
Basilisk
Based on the Basilisk.