IoIo was a priestess of the Roman goddess Juno. Juno was the jealous wife of Jupiter, the king of the gods. Jupiter was indeed very unfaithful. When Jupiter fell in love with Io, he transformed himself into the shape of a dark cloud to hide himself from his jealous wife.
However, Juno noticed the small cloud and suspected that the cloud was one of Jupiter's tricks. Thus, she approached to check the true nature of the cloud. As soon as Juno arrived, Jupiter immediately transformed Io into a white cow to avoid his wife's wrath. But Juno guessed the intrigue and asked if Jupiter wanted the cow as a gift. Jupiter could not refuse such a little gift without giving himself away.
Thus, Juno tied the poor cow and sent her faithful servant Argus to watch over Io. Argus had a hundred eyes and only a few were ever closed at any time. To free Io, Jupiter sent his son Mercury to sing and tell boring stories to make Argus sleep with all his eyes. Mercury told so many stories that finally Argus close all his hundred eyes. Only then did Mercury kill Argus and untie Io, who ran home free.
Yet when Juno discovered what had occurred, she was so furious that she sent a vicious gadfly to sting the cow forever. Moreover, to honor the memory of her faithful servant, Juno put the hundred eyes of Argus on the tail of her favorite bird, the peacock. The hundred eyes could not see any more but beautifully decorate the tail of the peacock.
Meanwhile, Io, who was still prisoner into the shape of a cow, could not get rid of the malicious gadfly. Finally, after Jupiter vowed to no longer pursue his beloved Io, Juno released Io from her inhuman prison, and Io settled in Egypt, becoming the first queen of Egypt.
The Jovian moon Io was named for the mythological character Io by Johannes Kepler, and Simon Marius. And finally, when Voyager 1 passed Io in March 1979 and imaged the surface, the image clearly showed the hoof print of a heifer!