In Greek mythology, Proteus (Πρωτεύς) is an early sea-god, one of several deities whom Homer calls the "Old Man of the Sea," whose name suggests the "first," as protogonos is the "firstborn." No mention is made of his parents, until for later mythographers he became the son of Poseidon in the Olympian theogony, or of Oceanus and a Naiad. and the herdsman of Poseidon's seals. He can foretell the future, but will change his shape to avoid having to; he will only answer to someone who is capable of capturing him. From this Proteus comes the adjective protean, with the general meaning of "versatile", "mutable", "capable of assuming many forms."
The myth of Proteus
According to Homer (Odyssey 4:412), the sandy island of Pharos situated off the coast of the Nile Delta was the home of Proteus, the oracular Old Man of the Sea and herdsman of the sea-beasts. At Pharos he welcomed Dionysus in the young god's wanderings. (The same island much later became the site of the lighthouse of Alexandria, one of the seven Wonders of the World, but that belongs to History.)
In the Odyssey, Menelaus relates to Telemachus that he had been becalmed here on his journey home from the Trojan War. He learned from Proteus' daughter that if he could capture her father he could force him to reveal which of the gods he had offended, and how he could propitiate them and return home. Proteus emerged from the sea to sleep among his colony of seals, but Menelaus was successful at holding him, though Proteus took the forms of a lion, a serpent, a leopard, a pig, even of water and a tree. Proteus then answered truthfully, further informing Menelaus that his brother Agamemnon had been murdered on his return home, that Ajax the Lesser had been shipwrecked and killed, and that Odysseus was stranded on Calypso's Isle.
Another story tells that at one time the bees of Aristaeus, son of Apollo, all died of a disease. Aristaeus went to his mother, Cyrene, for help; she told him that Proteus could tell him how to prevent another such disaster, but would do so only if compelled. Aristeus had to seize Proteus and hold him, no matter what he would change into. Aristeus did so, and Proteus eventually gave up and told him to sacrifice 12 animals to the gods, leave the corpses in the place of sacrifice, and return three days later. When Aristaeus returned after the three days he found in one of the carcasses a swarm of bees, which he took to his apiary. The bees were never again troubled by disease.
The children of Proteus include Eidothea, (the "very image of the Goddess"), whom Menelaus seduced, and Polygonos and Telegonos, who both challenged Hercules, got defeated and killed.
In Euripides tragedy Helen (produced in 412), the often unconventional playwright, who introduces a "real" Helen and a "phantom" Helen (who caused the Trojan War), gives a backstory that makes the father of his character Theoclymenus, Proteus, a king in Egypt who had been wed to a Nereid Psamathe. In keeping with one of his themes in Helen, Euripides mentions in passing Eido ("image"), another unseen daughter of the king. Euripides' king (never seen) is only marginally related to the "Old Man of the Sea"  (http://www.nottingham.ac.uk/classics/staff/LSF/Euripides/helen.html).
From his transforming nature, and mutifarious aspects comes our adjective "protean." A "protean career" would embrace many human concerns: Leonardo da Vinci was a painter, a sculptor, a scientist, a designer of fortifications: his career was "protean."
Gods of the Greeks, Carl Kerenyi
The Greek Myths: The Complete And Definitive Edition, Robert Graves
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Medieval Greece / Byzantine Empire