Io, a princess and a priestess of Hera, with whom Zeus was in love. To save the girl from Hera's jealousy, Zeus turned her into white heifer. But Hera didn't trust her husband and started to haunt and torment the poor Io, who in her attempt to save herself wandered till Egypt, where she gave birth to her son Epaphus and was later worshiped under the name of Isis. The place where she crossed from Europe to Asia Minor is still called Bosphorus ('cow crossing').
See: Correggio Zeus and Io.