In Greek mythology, Eidothea (Ancient Greek: Εἰδοθέα) is a sea goddess and daughter of Proteus, the Old Man of the Sea. Homer calls her "the very image of the Goddess" and makes her the guide who teaches Menelaus how to seize her shapeshifting father at Pharos so that he cannot escape without speaking true prophecy.
Adapted from Wikipedia, "Eidothea (mythology)" and "Proteus (Greek mythology)" · CC BY-SA 4.0
I.In the Odyssey
When hunger wasted Menelaus's men on the Egyptian coast, Eidothea met the king as he wandered apart from his comrades. Because her heart was moved above all others, she told him that at mid-heaven the unerring Old Man of the Sea, immortal Proteus of Egypt and servant of Poseidon, comes forth from the brine and sleeps among his seals in hollow caves.
She instructed Menelaus to seize her father and hold him through every form he took until Proteus answered without deceit. Through her counsel Menelaus learned that Odysseus still lived on Calypso's isle — the first sure news that would later reach Telemachus in Sparta.
II.Names and later tradition
Ancient sources give the daughter of Proteus several related names. Homer uses Eidothea; Euripides' Helen calls her Eido in youth and Theonoe ("god-minded") after she proves capable of prophecy; other traditions also know her as Eurynome.
Euripides sets a king Proteus of Egypt, already dead at the play's opening, as father of Theoclymenus and Theonoe and husband of the Nereid Psamathe. Scholars often distinguish this tragic king from Homer's immortal sea-god, though the two figures share Egypt, the sea, and their link to Menelaus's wanderings.
III.Homonym note
Eidothea is a common mythological name borne by other unrelated women, including an Oceanid nurse of young Zeus, a nymph of Othreis, and figures in Carian and Thracian genealogies. The daughter of Proteus who helps Menelaus must be kept distinct from these homonyms.