“Scrooge's Brother No. 1”, 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 Donald Duck and Della Duck, as well as another who fathered Fethry Duck. This is contrast to most accounts of Duck genealogy where Scrooge's sister Hortense McDuck was Donald and Della's mother while their father was Quackmore Duck, no relation of Scrooge's, and where Lulubelle Loon and Eider Duck (neither of them a blood relation of Scrooge's) were Fethry's parents.
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.