Man in the iron mask
Another hypothesis is that the prisoner was a illegitimate child of the Queen, Anne of Austria. The father would either be Cardinal Mazarin, or George Villiers, 1. Duke of Buckingham or one of the musketeers. Cardinal Richelieu would have arranged this meeting. This theory is less likely, however, because the child would have had no right to the throne, and imprisoning him would therefore have been no use.