King John of England ,

Who, John? John I "Lackland", King of England

He was a horny toad to begin with, and his grandson Edward I and even more so great-great Edward III were very prolific. (Edward II was...not so successful.)

All those younger sons and daughters had to marry *somebody*, and in most cases the only choice was to marry down. Give that a few generations, and you've gone from titled nobility to untitled gentry to middle class. "A million" descendants is probably hyperbole, but Geni has him tagged with a plausible 10,000 or so.

I would say that it is most likely true. A lot of my connections to "americans" goes via him.