Io was a character on Xena: Warrior Princess. She was an oracle, who put Xena through several tests to discover if she was willing sacrifice herself to save Prometheus. When she passes, Io tells her the location of the Sword of Hephaestus, which will break the chains that bind him.
In mythology Io was one of Zeus' lovers, who was turned into a cow to prevent Hera from finding out about her. Io eventually wound up in Egypt, where she wound up founding the line of the pharaohs. The entire myth appears to be an elaborate explanation as to why the Egyptians worshipped a cow-headed goddess.