From The Art and Popular Culture Encyclopedia
The Nehushtan (or Nehustan, Hebrew: נחושתן or נחש הנחושת), in the Hebrew Bible, was a sacred object in the form of a snake of brass upon a pole. The priestly source of the Torah says that Moses used a 'fiery serpent' to cure the Israelites from snakebites.
King Hezekiah (reigned 715/6 – 687) instituted a religious iconoclastic reform and destroyed "the brazen serpent that Moses had made; for unto those days the children of Israel did offer to it; and it was called Nehushtan."
The tradition of naming it Nehushtan is no older than the time of Hezekiah.