Proteus was the herdsman for Poseiden, Greek god of the sea. He had great powers of prophecy, and he lived in a huge cave and watched over his herds of sea calves. He prophesized only if seized. But no one could catch him because he would change himself into the shape of many other things. Changing his form, like the sea, was his means of avoiding capture. The only way he could be caught was to surprise him in his sleep.
Menelaus, king of Sparta, had exceeding difficulty in returning from Troy. His trip home took eight years. When he finally ran short of provisions, he was advised by a nymph that Proteus, if forced, could tell him the way home. Menelaus found the god in deep slumber, seized him, and held on despite Proteus’s successive transformations into a lion, a serpent, a leopard, a boar, water, and a tree. Proteus gave in and provided the information Menelaus wanted.
A person who can vary his attitude at will or is shifty and fickle is protean. In a complimentary sense, it may refer to someone blessed with versatile talents or a flexible nature.