Why is Io a cow in the Hamilton version of mythology

-Looked like a heifer but talked like a girl. Was mad. - Driven on a flight that never ends. Has horns on her head. - She stood still when Prometheus said her name. - Tried to hide Io by wrapping the earth in a cloud so thick and dark that night came. Hera went down to earth and ordered the cloud away. - Swore that he had never seen her and that she had just sprung from the earth. Hera told him to make Io a present for her and he made her into a cow. After Argus died Hera sent a gad-fly plague that drove her mad. Prometheus told her that the part of the sea she first ran along would be called Ionian and the Bosphorus which means Ford of the Cow would preserve the memory of what she went through. When she reached the Nile Zeus would turn her back into a human. She would bear him a son named Epaphus. Her descendant is Hercules.