Sandro Botticelli, Miracle of St. Eligius

1490-92
Tempera on panel
Galleria degli Uffizi, Florence

The Web Gallery of Art, source of this photograph, gives this explanation:

According to legend, St Eligius shod a horse that was possessed by devils. He cut one of the horses legs off, and after completing his work reattached it while making the sign of the Cross. The Devil, portrayed as a woman with horns, is watching the holy smith. A groom is having considerable difficulty controlling the impetuous horse.
The Gallery does not indicate the source of this legend, and I have not yet found one.

More of St. Eligius