Catfish McDuck was Scrooge's uncle.
Biography
Similar to Pothole McDuck, Catfish was an uncle of Scrooge's who moved to America in order to get rich, and subsequently became a steamboat captain on the Mississippi. Compared to Pothole however, Catfish was far less successful; his old, filthy, broken steamboat was barely able to sail, and Catfish smelled like… well, a catfish (that's why he had been nicknamed that !). All this heavily contrasted the image of himself that Catfish described in his letters to his brother who had stayed in Scotland, where he wrote he was fantastically wealthy and successful. These letters inspired young Scrooge to join Catfish's crew in Amercia. He arrived in time to help Catfish win a race against the dishonest Ribbit, of which the prize was the Cornpone Gables, a wealthy mansion. Thanks to Scrooge's help, Catfish won, but kept the mansion for himself, only paying his nephew 30 cents for his work.
Trivia
Catfish was created for DuckTales but heavily drives inspiration from the shadowy Pothole McDuck as cited in Barks's The Great Steamboat Race; aside from the name change, Catfish was actually a pretty plausible version of who this Pothole might have been. However, a few years after DuckTales came out, Don Rosa created a face and a history for Pothole, retconning that "Pothole" was just a nickname for Angus McDuck and creating Angus a face and a wealth that had nothing in common with DuckTales's Catfish. From that point forward, Catfish has been considered a different character from Barks's Pothole.