Argus was a servant of Hera's described as having a hundred eyes. He was thus a very effective watchdog, as only a few of the eyes would sleep at a time; there were always several eyes still awake. His last task for her was to guard a white heifer from Hera's hubby, Zeus. (Hera knew that the heifer was in reality Io, one of Zeus' many girlfriends.)
To free Io, Zeus had Argus slain. To reward good service, Hera had the hundred eyes of Argus preserved forever, in a peacock's tail.