Argos (or Argus) was a Greek hero, who had only one eye (another version gives him four eyes, yet another one gives him many eyes). He fulfilled many labors: freed Arcadia from a gigantic bull, killed Satyr, who harmed Arcadians, slew Echidna, the monstrous daughter of Tartarus. Hera employed him to watch over the white heifer Io, of whom she was jealous. Thanks to his many eyes he could watch Io day and night. Zeus asked Hermes (Mercury) for help. Having put Argos to sleep by magic, Hermes slew him. Willing to give immortality to her faithful servant, Hera moved his many eyes to the tail of peacock, sacred bird of Argos.
See: Peter Paul Rubens Argus and Mercury.
Diego Velázquez. Mercury and Argus.