What work can a painter never quite finish?

Leonardo da vinci gained the notoriety of not always finishing his paintings. But he finished some of them. There was 1 piece of art in particular that Leonardo never finished, and ached for. His horse. I don't remember what it was made out of, but some sort of medal. He had to leave it when an invasion attacked. Scientists say, when he died, it appeared he was still aching for his horse.