In one of my classes, we are learning about the history of science and technology and one of the medieval paintings we saw showed dragons on top of a castle. My question is how did the idea of a dragon/its significance in medieval come from? Also, dragon's arent unique just to Europe as there are other cultures (China) where they are mentioned too. Is there any relationship between the appearance of dragons in different locations?

Forgive me if my question is weirdly worded, I'm just curious about dragons.