u/CoeurdeLionne and u/sunagainstgold answered a similar question a few weeks ago:

Why do animals in Medieval manuscripts look so crazy? Did the painters not know what they looked like? Was it intentional? Did they have a hard time drawing?

There are also some good discussions on similar topics in the FAQ:

The FAQ about visual arts

Why did it take so long for artists to get to grips with perspective?

Why do professional paintings look photo-realistic in 1700s England yet look so awful 100-150 years earlier?

Why did pre-renaissance christian art degrade so much compared to the classical period?