A Roman Marble Head of Io, circa 2nd Century A.D.

turned slightly to her left, a pair of horns flanking the forehead, her wavy hair parted in the center and surmounted by a wreath of oak leaves and clusters of berries, a cluster of grapes falling over each side of the neck, a small head of a cow or bull emerging from the back; no restorations.

European private collection, by April 20th, 1979, when appraised in writing by Herbert Cahn, Basel
Döbritz, Frankfurt am Main, November 19th, 2016, no. 475, illus.

The horns and bovine head suggest a representation of Io, the Argive princess turned into a cow by Zeus or Hera, although known depictions of Io in the round show her only with horns (cf. B. Freyer-Schauenburg, Römische Mitteilungen, vol. 90, 1983, pp. 35ff.). 

The closest parallel for the highly unusual iconography of the small bovine head emerging from the back is a small Roman rosso antico portrait head of a boy also wearing a wreath of ivy berries and formerly in Berlin, Antikensammlung, inv. no. Sk 134 (http://arachne.uni-koeln.de/item/objekt/213510).

The iconography may have Egyptian origins; see the janiform black marble bust of Isis conjoined with Apis in the guise of a bull in the Vatican Museums, Museo Gregoriano Egizio, inv. no. 22807 (from the Villa Hadriana: The Vatican Collections: Papacy and Art, exh. cat., 1982, p. 180f., no. 97; LIMC, vol. 2, p. 181, no. 34, pl. 181). Io and Isis were occasionally combined in Roman sculpture (cf. Sotheby's, New York, June 3rd, 2015, no. 36).