Mr McDuck, sometimes known by the fanmade name of Peter McDuck, was an anthropomorphic duck.
Description
Little is known about Scrooge McDuck's great-great-great-grandfather in the paternal line, save that he would logically have been the nephew of the pirate Danblane McDuck and the moneylender Ebeneeza McDuck. Scrooge once recounted that he had been the one to originally buy the Heirloom Watch that would later find its way to Scrooge's great-uncle Quagmire McDuck, and that he had worked hard in a coal mine for the money it had cost him. At any rate, he ostensibly got married, and had at least one child, Seafoam McDuck. One account identifies his wife as a daughter of Cornelius Coot, which, absent time-travel shenanigans, makes little chronological sense unless one subscribes to the source in question's theory that Cornelius Coot lived in the 1600's rather than 1800's.
Behind the scenes
Scrooge's great-great-great-grandfather and Seafoam McDuck's father in 1999 in Johannes A. Grote's Duck Family Tree.
However, due to a translation error in the German version of The Heirloom Watch, Seafoam in Grote's tree is not Scrooge's great-great-grandfather but instead his great-great-great-great-grandfather. Thus, Grote includes both a father of Seafoam McDuck, and a great-great-great-grandfather for Scrooge in the paternal line, as two different characters.
The character who takes up the place of Scrooge's great-great-great-grandfather in Grote's tree, depicted as Seafoam's son, is accounted for on the Wiki at Mr McDuck (Seafoam's Relative). If Seafoam is replaced in his proper context, he obviously cannot both be Seafoam's son and Scrooge's great-great-great-grandfather; however, which of the two traits should be jettisoned is up to personal preference, and thus this McDuck may either be the same as the great-great-great-grandfather covered on this page, or the unrelated son of Seafoam McDuck and thus Scrooge's great-great-great-uncle.
At any rate, the 2008 story Hearts of Gold mentioned "Scrooge's great-great-great-grandfather" without giving him a name, establishing his early employment as a coal miner and his being the original owner of the Watch.