Wait, I definitely learned of the royal "we" in Latin. Specifically, this came up in Apollo and Daphne[1], with much of Apollo's first speech in the first-person plural referring exclusively to himself. We were taught in class that it was related to his arrogance to use a royal "we", which is a large theme of the story.
[1] http://en.wikisource.org/wiki/Daphne_and_Apollo