At the Art Institute of Chicago I saw two lovely depictions of donkeys. The first was this fragment from an Egyptian tomb with several donkeys on it. From an artistic perspective I thought this was one of the most beautifully rendered depictions of an animal I've ever seen.
There is a softness and subtlety to it in real life that does not come through in the snapshot.
The related object that I enjoyed seeing was the playful Greek wine cup, made in the shape of a donkey's head. Very clever!
Neither of these objects are in the Art Institute's online collection, but you can see other ancient (and modern) items from their collections at this website.