Ryujin
Legend
I thought the seventh son of a seventh son thing was strictly a bit of Irish folklore, but Wikipedia turned up this fascinating little bit of lore.It predates Pratchett by centuries and sports in multiple mythologies across the works. It’s an old folklore thing.
All you need to know is he’s the first Time Lord and he’s a baddie. If it bothers you that he appeared before, just pretend he hasn’t. Even as a new character that basic story works just as well as some other random new character called Caprica or something would.
In Lancashire and particularly in Blackburn there was, in the eighteenth and nineteenth centuries, a tradition of calling seventh sons of seventh sons (and seventh sons) 'Doctor' (forename) because of their supposed abilities as healers.