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Grandpa Ostman (Opa Ostman in the original German) is or was a male anthropomorphic duck.
According to one account, the “Grandpa” whose old buggy Donald Duck borrowed in 1943 to take part in the Screwball Race was not Donald's Grandpa Duck (who was, by then, likely deceased) but another old man, Grandpa Ostman. It is suggested that Ostman was not Donald Duck's grandfather but Huey, Dewey and Louie Duck — supposedly as the father of their mother, though as it is commonly held that the triplets' mother was Donald's sister Della, it is more likely that Ostman was the father of the children's own unseen father.
Behind the scenes
Donald and the nephews borrowed “Grandpa's” horse in The Hard Loser (1943).
At the time, Carl Barks likely intended for his “Grandpa” to be Donald Duck's own granfather, but it was soon established that Grandpa Duck was deceased in the "present day". Although one could still assume that he died somewhere in-between 1943 and the first stories featuring an explicitly widowed Grandma Duck, the 1976 German localization of Barks's vintage tale provided another possibility by giving “Grandpa” a last name other than “Duck”: “Ostman”.
Following this lead, Johannes A. Grote's Duck Family Tree retconned Barks's “Grandpa” to be Huey, Dewey and Louie Duck's grandfather on the non-Duck side, though due to another peculiarity of Grote's Duck genealogy, this makes him the father of Della Duck rather than of the triplets' father as one would more sensibly assume — though even that is not without its difficulties, as one is then forced to come up with reasons why Huey, Dewey and Louie kept their mother's last name.