According to legend, Zeus fell in love with Io, beautiful young priestess of Argive Hera, thus provoking the wrath of his jealous divine wife. To avoid her revenge, Zeus transformed Io into a gleaming heifer, but Hera required - as a demonstration of loyalty - that Io were consecrated to her and placed in the custody of the "all-seeing" Argos, described as having as many as one hundred eyes. He was a descendant of Zeus, with prodigious strength, able to see in all directions and sleep with his eyes closed halfway.
To restore freedom to his beloved, Zeus instructed Hermes to neutralize Argos. Thus Hermes either killed the monster or made him sleep with the sound of Pan's flute or with his own divine wand.
As a sign of gratitude for the help anyhow received, Hera immortalized Argos in the constellation of the Peacock, placing his hundred eyes on the feathers of the bird sacred to her.