In Greek mythology, Io (Ancient Greek Ἰώ) was an Argive princess and priestess of Hera, one of Zeus's mortal lovers. Transformed into a white heifer to hide her from Hera, she was guarded by Argus Panoptes, tormented by a gadfly, and driven across Europe and Asia until she reached Egypt, where Zeus restored her form and she bore Epaphus. She is the mythic ancestress of lines that include Danaüs, Heracles, Perseus, Cadmus, and Dionysus — not Jupiter's volcanic moon, which was named after her in 1614.
Adapted from Wikipedia, "Io (mythology)" · CC BY-SA 4.0
I.Family
Most sources make Io the daughter of the river-god Inachus of Argos; alternatives name Iasus or other Argive kings as her father. Zeus was her lover; their son Epaphus became an Egyptian king in Greek genealogical lore. Through Epaphus and his descendants (Libya, Belus, Agenor, Danaus, and others) Io stands at the root of major Argive, Theban, and heroic stemmata.
II.Mythology
Zeus desired Io, a priestess of Hera. Either Zeus or Hera transformed her into a white heifer; Hera then claimed the cow and set the hundred-eyed giant Argus Panoptes to watch her. Hermes slew Argus at Zeus's command (Hera afterward set Argus's eyes in the peacock's tail). Hera next sent a gadfly that drove Io mad across the world.
Her itinerary — treated at length in Aeschylus' Prometheus Bound and in Ovid — gives names to the Ionian Sea and the Bosphorus. She finally reached the Nile, where Zeus restored her human shape by a touch and she gave birth to Epaphus. Greek authors equated aspects of her Egyptian sojourn with local cow-goddess and Apis traditions without erasing her identity as the Argive heroine.
Parentage of her father and details of who performed the metamorphosis vary among Apollodorus, Ovid, Hyginus, and the tragedians; the cow-wandering, Argus, gadfly, and Egyptian restoration form the stable core.
III.Namesakes
The atlas id io is exclusively the Argive mortal beloved of Zeus. Jupiter's moon Io (named by Simon Marius in 1614), the Ionian Islands as a geographic label, and other scientific or cultural namesakes are later commemorations of the myth — not the character herself, and not to be merged with unrelated figures who share short Greek names.