Ceroessa (Keróessa)

According to the tradition summarized in available sources, Ceroessa was born to Io (here transformed into a heifer) and Zeus. One version (via Hesychius as reported in later summaries) places her birth on the banks of the Golden Horn by the altar of the nymph Semystra; she was reared by Semystra. In another tradition (preserved in Nonnus' account as cited), her birthplace is aligned with that of her brother Epaphus in Egypt. Ceroessa later lay with Poseidon and bore Byzas (and in some mentions a son Strombos), and Byzas is said to have founded Byzantium and named the Golden Horn (Greek Χρυσόκερας) after his mother.

The sources provide only limited physical detail: she is said to have grown up surpassing other local maidens in beauty. No further iconographic or detailed bodily description is attested in the cited summaries.

No supernatural powers or magic are ascribed to Ceroessa in the available accounts. Her functional role in the myth is genealogical and etiological: as daughter of Io and Zeus and mother of Byzas by Poseidon she connects gods, heroic ancestry, and the foundation and place-name of Byzantium. Some traditions add kin (brother Epaphus) or another son (Strombos), but these affect lineage rather than confer supernatural abilities.