Lady with an Ermine, 1485 by Leonardo de Vinci.

This work is one of only three portraits of women done by Leonardo and has sustained a lot of retouching and overpainting throughout the years. However the woman (whose identity remains in dispute) and the ermine itself are so wonderful they are both considered to have been painted by da Vinci. The ermine has actually been identified as a ferret. Seized by the Nazi's in WWII the painting was returned to Poland in 1946.