Io is an ancient figure, and a significant character in Greek tragedy:
As Io tells her own story in Aeschylus' Prometheus Bound, she rejected his whispered nighttime advances until the oracles caused her own father to drive her out into the fields of Lerna. There, Zeus covered her with clouds to hide her from the eyes of his jealous wife, Hera, who nonetheless came to investigate. In a vain attempt to hide his crimes, Zeus turned himself into a white cloud and transformed Io into a beautiful white heifer. Hera was not fooled. She demanded the heifer as a present. (Wikipedia)
The lore involving Io is huge, as the entry on her at Theoi reveals. She was a Naiad, a daughter of the river Inachus. He supposedly introduced the worship of Hera to Argos.
Aeschylus was fascinated by the figure of this tormented woman who was raped by Zeus, transformed into a wandering heifer by Hera; she wanders onstage in Prometheus Bound, and is given a preview of her long travails by the tortured Titan. In a sense, she and Prometheus see in each other the mirror of their own sufferings at the hands of the Zeus-Hera marriage.
Io's descendents, the Danaids, are recalled in The Suppliants. And curiously, the image of Io appears on the shield of Turnus, the ruler of Italy, favored by Hera/Juno, and the ultimate antagonist of Aeneas.
Io's son Epaphus was a distant ancestor of Perseus, and according to Aeschylus, of Herakles, who would at long last liberate Prometheus.