Sculptural or painted decorations based on antique Roman prototypes used for the ornamentation of grottoes, hence their name. They are usually composed of human, plant, or animal forms that can be either fantastic, playful, or ominous. This type of decoration was reintroduced in the 1490s when Emperor Nero's Domus Aurea Golden House in Rome, decorated with fantastic grotesques rendered in brilliant colors and gilded, was rediscovered. Several artists, including Pinturicchio, Alexander VI's favorite painter, lowered themselves with ropes to view the Neronian decorations. Pinturicchio then decorated Alexander's apartments in the Vatican Palace with grotesque ornamentations (1492-1494).
Historical dictionary of Renaissance art. Lilian H. Zirpolo. 2008.