The Black Family Tree mentions a Charlus Potter who was married to Dorea Black (1920-1977), a grandchild of Phineas Nigellus Black (BFT). They had one son (unnamed) (BFT).
Charlus Potter is part of the Potter family, but shows up on the Black Family Tree because he married Dorea Black.
For years, fans assumed that Charlus is Harry's grandfather. However, this seemed unlikely because Jo has told us that "James's parents were elderly, were getting on a little when he was born, which explains the only child, very pampered, had-him-late-in-life-so-he's-an-extra-treasure, as often happens, I think. They were old in wizarding terms, and they died." (TLC) Dorea was only 57 when she died. When Rowling published her history of the Potter family on Pottermore, she put this matter to rest by writing about Euphemia and Fleamont Potter, James' parents (Pm).