Back in the ancient old days, when idol worship was conflated with king worship, basically anything could be an idol. Any material, shaped or unshaped, could be worshipped.
In the United States, it is not traditional to worship idols of wood and stone. No one erects statues to deify World War II veterans, to whom their descendants might pray for success in future wars. Instead, statues are put up as a remembrance only, without ascribing any power to them. Monotheism has made a lot of inroads in the world in the past few thousand years.
But in Babylon, and generally in ancient times, there were idols for everything. Most of these idols were in the form of statues.
There were idols for preventing theft (which, of course, had to be protected from theft) and idols for making kids behave (which, incidentally, were kept far away from children.)
There was an idol people threw rocks at to serve. Everything in the natural world was idolized. Literally. There was even an idol that was served by pooping in front of it, called baal peor.
Making a new religion or idol was a very normal and royal thing to do at the time, kind of like founding a charity nowadays, and the nice thing about being the king was that he could make up whatever he wanted. So, in the third chapter of the Book of Daniel, instead of creating the Nebuchadnezzar Family Nonprofit Organization to Support Chaldean Orphans, Nebuchadnezzar began working on a golden idol of forever, which would symbolize his power and might.