Gallery of Art, source of this photograph, gives this
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 been able to find one.
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