Why are there concepts of dragons and giants in various cultures around the world? Because people have been discovering fossils of megafauna for thousands of years. And there had to be some explanation for these giant creatures.
“Fifty-five years ago, a French geologist called Josue-Heilmann Hoffet went to Indochina to conduct a geological survey and found dinosaur bones. Near the border between Vietnam and Laos there is a very small village in a dry teak forest. The people of the village are Qatang and they are animists. Hoffet arrived in this small village, and when he began observing the rocks, the people say, ‘Oh, you must be looking for the stone bones-of the sacred buffalo.’ They told Hoffet that when the sacred buffalo are young, they carry the sun in the sky each day, and when they get old, they die in this place, not far from the village. This is their legend. They took him to see the big vertebrae of the sacred buffalo. They were dinosaur bones. Hoffet was fascinated and wanted to collect the bones, but they told him, ‘You cannot touch these vertebrae because they are sacred. If perhaps you do a sacrifice, maybe you can collect some of them.’ So he paid them a buffalo, a young buffalo, and he was allowed to collect some of the fossil bones. He described these bones in 1936 in Hanoi in a small paper, ‘Description of New Titanosaurians in Bas-Laos.’ (Hunting Dinosaurs by Louie Psihoyos and John Knoebber (page 132)]
"A number of European caves, especially in central Europe, traditionally maintain the name of the dragon’s (or dragons’) cave or lair. During the 17th century, the German doctor Petersonius Hayn found some large skulls, isolated teeth, and bones in several caves in the Carpathian Mountains around Moravia. In 1673, Hayn had a article published by the Halle Academy of Sciences entitled “Skulls of Dragons in the Carpathians.” Around the same time, another German, named Vette, found similar remains in Transylvania. According to their discoverer, these bones belonged to flying dragons. Illustrations of the material described by Hayn and Vette still exist. In both cases, they are of Quaternary cave bears, a powerful animal that was one of the largest carnivorous mammals. The famous Austrian paleontologist Othenio Abel analyzed the legend of the dragon of Klagenfurt (Austria) early in the 20th century. At the beginning of the 14th century in the spot known as the “Dragon’s Grave,” the skull of a woolly rhinoceros from the ice age was found and was subsequently exhibited in the city’s town hall. This specimen served as a model for the sculptor Ulrich Vogesland for this creation of a statue of a dragon, which today is an emblem of the city of Klagenfurt" (Starring T. rex!: Dinosaur Mythology and Popular Culture by Jose Luiz Sanz, pg. 122-123).