Scrooge McDuck

Scrooge McDuck was, according to the Ninth Doctor, a "peasant" compared to Drake Ayelbourne, the richest man in the universe. (COMIC: Mystery Date)

After the Twelfth Doctor sarcastically told her that he had whole rooms in the TARDIS "brimming" with money, Bill Potts cited him as having "kind of a Scrooge McDuck thing going on", which the Doctor brushed off as coincidental. (PROSE: Inflicting Christmas)

Behind the scenes
Scrooge McDuck is a Disney character who first appeared in various comic books from 1947 before going on to appear in other media. He is the in-universe uncle of Donald Duck (from the same fictional universe as Mickey Mouse). The character is known for, among other things, his immense wealth.

David Tennant voiced an alternative Scrooge in the 2017 reboot of the animated series.

While unspecified in any DWU material, Scrooge speaks with a Scottish accent, which may have been one of the factors that prompted Bill to compare the Twelfth Doctor to the character.

One of the most renowned authors of comics featured Scrooge, American cartoonist Keno Don Rosa and self-confessed fan of Doctor Who, had drawn a rendition of Scrooge (in a costume reminiscent of the Eleventh Doctor's) using the TARDIS to go back in time to 1896 in the Klondike, meeting his younger self just as he is about to become a millionaire, and another one where Scrooge (in his normal costume) appears intent on selling the TARDIS at a "Slightly Used Tardis Lot". Both cartoons are considered canonical on the, though not on Tardis Data Core.

IDW Publishing, incidentally, would go on to be the publisher of multiple Disney comics in the United States, including a series starring Scrooge.