Hera almost caught Zeus with a mistress named Io, avoided by Zeus turning Io into a beautiful white heifer. However, Hera demanded that Zeus give her the heifer as a present.
Once Io was given to Hera, she placed her in the charge of Argus to keep her separated from Zeus. Zeus then commanded Hermes to kill Argus, which he did by lulling all one hundred eyes to sleep. When Hera learned of Argus’ death, she took his eyes and placed them in the plumage of the peacock, accounting for the eye pattern in its tail.