Nabonidus was the last king of Babylon, reigning from 556 to 539 B.C.E., and though he isn't mentioned in the Bible, many experts believe he was the real Babylonian king who went mad and acted like an animal rather than Nebuchadnezzar.
According to Daniel 4:25, Nebuchadnezzar, had a disturbing dream which his interpreter Daniel told him meant, "You will be driven away from people and will live with the wild animals; you will eat grass like the ox and be drenched with the dew of heaven. Seven times will pass by for you until you acknowledge that the Most High is sovereign over all kingdoms on earth."
So said, so done. One day Nebuchadnezzar was bragging about his greatness; the next, he was driven from his home, living with wild animals and eating grass. Seven years later, he recovered his sanity and praised God [source: Easton's Bible Dictionary].
But numerous Babylonian writings and other ancient texts -- including the Dead Sea Scrolls -- make it clear that Nabonidus was the king with the unsound mind. So why the change? Some scholars believe that it's due to mistakes in the translation. Others think that it was a deliberate choice on the part of the editors of Daniel to better advance their ideals. Nebuchadnezzar was a very powerful king who destroyed the first temple in Jerusalem, so if the story was about him instead of Nabonidus, it's one of punishment and redemption [source: Bledsoe].