Donald Junior (name uncertain) is a peculiar anthropomorphic duckling.
According to one account, Donald Duck and Daisy Duck settled down together at one point in the 1950's and had a son, who, as a toddler, displayed the same impish temper that his cousins Huey, Dewey and Louie had at his age.
Behind the scenes
Donald Junior is an oddity who has a small role in the 1959 cartoon How to Have an Accident at Work.
It is obviously hard to reconcile his existence with most accepted canon, as in no stories posterior to 1940 do Donald and Daisy ever hint at ever having committed to their relationship to the point of having a child, instead continuing to refer to themselves as "girlfriend" and "boyfriend". The comic story A “What-If” Love Story of Imaginary Proportions (1987), in particular, clearly portrayed Donald and Daisy settling down and raising children of their own as a "what-if" scenario about which Donald has a nightmare.
Donald Junior was not explicitly named as such, the name being technically fanmade. In possible allusion to this Donald Junior, the 2014 Mickey Mouse Clubhouse episode Donald Jr. introduced a character of the same name, a young lookalike of Donald whom Goofy dubs "Donald Jr". Despite misreporting to this effect, the 2014 Donald Jr. is explicitly not the return of Donald's son, but a separate character.