Io
Ἰώ
A woman turned into a cow to escape Hera's eyes, doomed to wander the world.
Zeus changed her into a white heifer to keep Hera from finding out.
Hera asked for the cow as a gift and set Argus, the hundred-eyed watchman, to guard her.
When Hermes lulled Argus to sleep and killed him, Hera sent a gadfly that chased Io all the way to Egypt.
SourcesOvid, Metamorphoses