Louis XVII would have been King of France after the beheading of his father Louis XVI but for the fact that France was a republic at the time. He died while imprisoned in Paris; or maybe he didn't. The case is puzzling to this day, and conflicting evidence is offered from likely or unlikely sources.
San Marino is the oldest existing Republic in the world. This is not just some boast, but historical fact. The beginnings of San Marino, as befits any good story, are shrouded in legend, putting the founding of the city into the hands of a saint, St Marinus. Like most of the early and many of the later Catholic saints, he is mythical but features a nice story.