“Scrooge's Brother No. 2”, presumably named Mr McDuck, is or was an anthropomorphic duck.
According to one odd account, Scrooge McDuck had a brother who got married and became the father of Fethry Duck. This is contrast to most accounts of Duck genealogy where Lulubelle Loon and Eider Duck (neither of them a blood relation of Scrooge's) were Fethry's parents, although it is possible that Mr McDuck's offscreen wife was Lulubelle even so. This would also presumably make Mr McDuck the father of Dugan Duck's mother, even though sources do not mention this.
Behind the scenes
This mysterious sibling of Scrooge appears only in the 1980 Finnish Duck Family Tree, and is obviously assumed by most other sources not to exist. Scrooge does have more than one brother in other sources (Gideon McDuck being the most notable), but none of them are ever described as Donald Duck or Fethry Duck's father, and, indeed, none of them are known to have children at all.