A Comm for Historymemes

I think you got it! There has been quite extensive traffic between Southern and Central Europe in the Middle Ages due to trade, pilgrimage and warfare, so there were people who had seen actual turtles and brought back first hand accounts. A medieval monk should have known someone who's second cousin twice removed met someone who saw a turtle during pilgrimage.

Pangolins OTOH are native to Subsaharan Africa, India and Eastern Asia. Except of Marco Polo no European has travelled to Pangolinland during the Middle Ages. Stories did travel along the Silk Road, though, and and this drawing does resemble a pangolin much more than a turtle.