Proteus

Proteus is a prophetic sea-god, a herdsman of Poseidon's seals, famous for his power to change into any shape — beast, water, or flame — to escape those who would question him; only by holding him fast through every transformation could one win his truthful prophecy. From him comes our word 'protean'.

Because he could become all things while being, in himself, no one fixed thing, the allegorists made Proteus a symbol of the formless first matter that underlies the world and can receive every shape — the mutable substrate that yields its secret only to the mind that holds firmly to it.