These creatures are probably based on real, living dinosaurs that are described correctly, or they have been described with the combined features of several dinosaurs, or they may be based on a real dinosaur with additional exaggerations …
“The dragons of legend are strangely like actual creatures that have lived in the past. They are much like the great reptiles which inhabited the earth long before man is supposed to have appeared on earth. Dragons were generally evil and destructive. Every country had them in its mythology.” 
In the 1500′s a European scientific book called Historia Animalium, listed several dragons as still living but rare. 
And the Vikings carved dragons as their figure heads on the front of their ships. 
In 1572, Ulysses Aldrovandus heard that a peasant had killed a small dragon in northern Italy. So he got the carcass of the dragon, and put it in a museum.
The first dinosaur fossils were probably found in China as early as the fourth century.
These fossils were referred to as dragon bones.
 Knox, Wilson, Dragon, The World Book Encyclopedia, vol. 5, 1973, p. 265
 Gesner, Konrad, Historiae Animalium, 1551-1587, p.224
 ‘Exploring the Vikings’ Google Books
 Saint George and the Dragon
 Aldrovandus, Ulysses, The Natural History of Serpents and Dragons, 1640, p.402. Also see ‘Frontiers of Zoology’