Because we have found huge species of prehistoric turtle fossils. Throughout history humans have come across fossils and created myths based on these discoveries. I assume dragons were based on dinosaur bones and so forth.

https://th-thumbnailer.cdn-si-edu.com/fZqJ6mG_gB9ryReh4csyra5E1bA=/fit-in/1072x0/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer/72/72/72722d49-3bd6-41ad-913f-cd0e31c07c73/223993_web.jpg