The English translation available online on various libertarian websites (like that of the Ludwig von Mieses institute) is beyond attrocious. They seem to believe the comedian dramatist Terence was a historian and his braggart soldier character was a historical figure that he had something to report about. It's only natural they barely sourced any of the claims made by la Boétie. However I also have two scholarly translations (not into English) in which this particular claim is not sourced either. In English this whole passage goes like this:

The kings of the Assyrians and even after them those of the Medes showed themselves in public as seldom as possible in order to set up a doubt in the minds of the rabble as to whether they were not in some way more than man, and thereby to encourage people to use their imagination for those things which they cannot judge by sight. Thus a great many nations who for a long time dwelt under the control of the Assyrians became accustomed, with all this mystery, to their own subjection, and submitted the more readily for not knowing what sort of master they had, or scarcely even if they had one, all of them fearing by report someone they had never seen. The earliest kings of Egypt rarely showed themselves without carrying a cat, or sometimes a branch, or appearing with fire on their heads, masking themselves with these objects and parading like workers of magic. By doing this they inspired their subjects with reverence and admiration, whereas with people neither too stupid nor too slavish they would merely have aroused, it seems to me, amusement and laughter.

Does anybody know which of the ancient historians or his contemporary writers de la Boëtie is drawing on here? Is it an invention of his own?