

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).