What is 'Nehushtan'? Is that some sort of ancient idea of a god or diety worshiped at that time?
"He removed the high places and broke the sacred pillars, cut down the wooden image and broke in pieces the bronze serpent that Moses had made; for until those days the children of Israel burned incense to it, and called it Nehushtan" (II Kings 18:4).
Nehushtan is the name the people gave to the bronze serpent when they began worshiping it as an idol. The name is a play of words in Hebrew being a combination of the word for serpent, nachash, and bronze, nechosheth. It was a unique name for this particular idol.
Originally, the bronze serpent was made by Moses at the command of God. "Then they journeyed from Mount Hor by the Way of the Red Sea, to go around the land of Edom; and the soul of the people became very discouraged on the way. And the people spoke against God and against Moses: "Why have you brought us up out of Egypt to die in the wilderness? For there is no food and no water, and our soul loathes this worthless bread." So the LORD sent fiery serpents among the people, and they bit the people; and many of the people of Israel died. Therefore the people came to Moses, and said, "We have sinned, for we have spoken against the LORD and against you; pray to the LORD that He take away the serpents from us." So Moses prayed for the people. Then the LORD said to Moses, "Make a fiery serpent, and set it on a pole; and it shall be that everyone who is bitten, when he looks at it, shall live." So Moses made a bronze serpent, and put it on a pole; and so it was, if a serpent had bitten anyone, when he looked at the bronze serpent, he lived" (Numbers 21:4-9). The people had kept the casting for over a thousand years, but eventually people began to revere it, not as history but as an idol. Therefore, it had to be destroyed.