In Greek mythology, the goddess Hera transformed the nymph Io, Zeus's lover, into a cow. To guard Io, she appointed the hundred-eyed guardian Argos Panoptes. When Hermes killed Argos, Hera took his eyes and placed them in the peacock's feathers. The peacock thus became Hera's sacred animal. Its "thousand eyes" in the wheel are reminiscent of Argos and represent vigilance, omniscience, and immortality.