Problem: Daniel records that Nebuchadnezzar suffered from boanthropy (i.e. believing that you are an animal). Is this plausible? If this is the case, then why don’t extrabiblical sources record such an extravagant event?
Soultion: For one, we shouldn’t expect to discover any sources on this event. After all, ancient kings were not in the business of recording embarrassing failures or weaknesses like this. Moreover, most ancient events were never recorded or didn’t survive the tides of time.
In particular, records of Nebuchadnezzar’s life become very sparse in the latter half of his reign. OT scholar Paul Ferguson writes, “Meticulous historical records are available up to about the eleventh year of Nebuchadnezzar’s reign, after which the chronicles are practically silent.” Furthermore, Nebuchadnezzar’s religious interest begins to wane during the latter half of his reign as well.
All of this being said, there is fairly recent extrabiblical evidence of Nebuchadnezzar’s insanity. In 1975, the Assyriologist A. K. Grayson published this Babylonian cuneiform tablet from the British Museum. The text is fragmentary, and scholars debate its meaning. However, the text reads as follows (cited in archaeologist Siegfried H. Horn’s article, “New light on Nebuchadnezzar’s madness.”
2 [Nebu]chadnezzar considered
3 His life appeared of no value to [him, ……]
5 And (the) Babylon(ian) speaks bad counsel to Evil-merodach [….]
6 Then he gives an entirely different order but [. . .]
7 He does not heed the word from his lips, the cour[tier(s) – – -]
11 He does not show love to son and daughter [. . .]
12 … family and clan do not exist [. . .]
14 His attention was not directed towards promoting the welfare of Esagil [and Babylon]
16 He prays to the lord of lords, he raised [his hands (in supplication) (. . .)]
17 He weeps bitterly to Marduk, the g[reat] gods [……]
18 His prayers go forth to [……]
Commenting on this inscription, Paul Ferguson writes, “For some unspecified reason the king becomes extremely disoriented. His orders are contradictory, and he does not even heed the mention of his name. He does not show concern for son or daughter and ceases his care for worship centers. Even his own life is of no value to him. The text ends with the king going to the holy gate and weeping bitterly to the great gods. The text is much too small and fragmentary to dogmatically assert that it is the Babylonian version of the account in Daniel 4. It does, however, indicate that a great deal of caution is in order before dismissing the account of the king’s madness as nothing more than folklore.”
To be clear, this manuscript does not prove Nebuchadnezzar’s madness, but it does corroborate the biblical account in an interesting way.
 Paul Ferguson, “Nebuchadnezzar, Gilgamesh, and the ‘Babylonian Job,’” Journal of the Evangelical Theological Society (37/3, September 1994), 322.
 A. K. Grayson, Babylonian Historical-Literary Texts. “Toronto Semitic Texts and Studies,” No. 3 (Toronto: University of Toronto Press, 1975), pp. 87-92.
 Paul Ferguson, “Nebuchadnezzar, Gilgamesh, and the ‘Babylonian Job,’” Journal of the Evangelical Theological Society (37/3, September 1994), 323.