Interestingly, no. There’s never been one who wasn’t, but there’s no actual law that says so. Some people floated the idea of appointing Donald then impeaching and convicting Biden and Harris on some bogus charge (or assassinating them, but that was usually an unsaid “wink wink, nudge nudge”) to get him back in. Musk should be out of the line of succession for President at least if it does happen though since unless I’m mistaken the “natural born citizen” part of the requirements still stands