What hundred-eyed giant was set to guard Io?

The hundred-eyed giant set to guard Io was Argus Panoptes.

Argus Panoptes, whose epithet means “all-seeing,” served Hera as an exceptionally vigilant guardian. Zeus had transformed Io, one of his mortal or divine companions depending on the version, into a heifer to conceal their relationship. Hera nevertheless recognized the deception and entrusted Io to Argus.

Ancient sources do not agree on Argus’s exact appearance or parentage. Some describe him as earth-born, while others name Arestor, Inachus, or other figures as his father. The familiar count of one hundred eyes is especially associated with later literary tradition, including Ovid; earlier stories may simply emphasize his relentless watchfulness.

Zeus sent Hermes to rescue Io. Disguised as a shepherd, Hermes lulled Argus to sleep with music and storytelling before killing him. Hera then placed Argus’s eyes on the peacock’s tail, explaining a familiar mythological symbol. Argus is distinct from Geryon, another multi-bodied giant, and from Polyphemus, the one-eyed Cyclops.