Who is Proteus?

Proteus, the Old Man of the Sea, is a sea-god living on the island of Pharos. As Menelaus tells Telemachus, he and his crew were stranded there on their return home from the Trojan War. Proteus's daughter, Eidothea, tells Menelaus that if he wants to see home again, he will have to catch her father and force him to reveal which of the gods he offended in order to avoid being stranded on the island.
This is easier said than done, however. Proteus is a shape-shifter who can take on the appearance of many different animals and inanimate objects. But Menelaus and his men have little choice, so they lie in wait until Proteus appears. When he finally does so, they pounce on him, trying desperately to hold him down. Proteus is not going down without a fight, however, and he changes into a lion, a snake, a pig, a leopard, a tree, and even water. Somehow, Menelaus and his men manage to hold on until Proteus gives up the struggle.
Once everything has calmed down, Proteus tells Menelaus that he is stranded on Pharos because he did not make the appropriate sacrifices to Zeus before he set sail. What he must now do is make good on the omission and head to the river Nile, where he must do what he should have done in the first place. Only when he has made the sacrifice there will Menelaus and his men be able to leave Egypt and head home.