Whom did Hera get to guard lo?

Hera got Argus Panoptes to guard Io. Argus was a giant with many eyes, making him an effective watchman. Hera assigned him this task to keep Io, a mortal woman whom Zeus had turned into a cow to hide from her jealousy, under constant surveillance. Ultimately, Hermes was sent by Zeus to rescue Io and put an end to Argus's watch.