Potcrack McDuck (also known as Potluck McDuck) was an anthropomorphic duck.
Description
So called because he was not quite as stingy as the other McDucks would have liked, Potcrack McDuck was the son of Seafoam McDuck and Scrooge McDuck's great-grandfather. Born in the early age of inventing, Potcrack lived in Glasgow and invented a steam-powered bagpipe, which he called the Steam-o-Pipe. The machine's cacophonic music was not to the tastes of the time, and Potcrack never found a buyer; however, his neighbor James Watt actually copied Potcrack's Steam-o-Pipe when inventing his own steam engine. Potcrack later married with an unknown woman and had several children, including Titus McDuck, who would become Scrooge McDuck's grandfather.
At some point afterwards, Potcrack heard that gold had been found in the brand new United States of America. He decided to move there and become a prospector; although he dug a large mine, he never found any gold, and instead used his mine as a storage facility for all sorts of old junk he collected. His hear and great-grandson Scrooge McDuck eventually inherited the mine, where he actually found Spanish doubloons hidden amongst the junk.
Behind the scenes
The portrait of "Potcrack" in its original setting and in Grote's tree.
Potluck McDuck first appeared in January of 1974 in the story Old Potluck's Payoff (never yet published in English) drawn by Tony Strobl. The character was then used in 1992 by Don Rosa in The History of Clan McDuck, where he was called Potcrack McDuck.
In Johannes A. Grote's Duck Family Tree, a portrait of an old duck seen on Donald Duck's wall in Carl Barks's Going Ape is identified as Scrooge's great-grandfather, father of Titus McDuck. As the duck does indeed resemble Potcrack as drawn by Don Rosa, there is no reason not to consider this to be canon.