Phooey Duck is a supernaturally-created anthropomorphic duckling.
Brought into existence by a freak phenomenon when super-charged lightning struck Huey, Dewey and Louie Duck, Phooey Duck is functionally a "fourth triplet", named for his first words as he came into existence.
Identical to any one of the original trio, Phooey shares their personality, and also whatever knowledge they possess at the time of his materializing — for rather than existing perennially, he periodically "hiccups" into existence periodically in the vicinity of his "brothers", and never lingers for more than a minute. On several occasions, Phooey has helped his brothers save the day with quick thinking. Phooey's existence is not well-known, for obvious enough reasons.
Behind the scenes
Phooey first appeared in 1959 in the story Mastering the Matterhorn in a one-panel cameo; indeed, the nature of Phooey is originally an artist's mistake or error, referring to any time an artist accidentally draws four nephews instead of three in one of the panels.
After what may be the first two intentional appearances by Phooey in two game pages, The Shadow and Magical Mix-Up, an in-universe backstory was created for him in the 1999 story Much Ado About Phooey. His yellow outfit resembles that worn by Louie in some early cartoon appearances.
The 2017 Continuum's version of Phooey Duck is first referenced in the episode, What Ever Happened to Donald Duck?, where Dewey Duck named a fourth egg in an old family photo "Phooey Duck". The fourth egg turned out to be a mustard stain. Phooey Duck made a brief appearance in a 2019 episode of DuckTales 2017 entitled A Nightmare on Killmotor Hill in a dream sequence by Dewey Duck.