Yukk! (formally stylized with an exclamation point) is a fictional crime fighter who debuted on American television, as a semi-regular feature of a cartoon series entitled The Plastic Man Comedy/Adventure Show, which itself aired from September 1979 to September 1980.
Yukk! is an anthropomorphic oversized dog who serves as Mighty Man's sidekick; in the show, he played the part of comic foil to the more serious Mighty Man, said comic relief appearing in the form of the miniature doghouse that he constantly wore over his head. The purpose of the doghouse was to protect the world from his face, which was supposed to be the ugliest thing the world had ever known (and which was never seen by the viewers).
In addition to being his headgear, Yukk!'s doghouse also served as storage for the Mighty Phone (which allowed the mayor to contact Mighty Man) and the Mighty Machine (a pistol that transforms Brandon Brewster into Mighty Man, and back again). Other items could also be found inside the doghouse.
Yukk!'s "superpower" isn't really anything of the kind: he can create various destructive effects merely by removing his doghouse and causing the "target" to see his face, or by someone entering the doghouse (which happened more than once). As a comedic effect, Yukk!'s face was shown to be capable of not only making living creatures run away in terror, but could also destroy inanimate objects and wreck machinery that was invulnerable to even Mighty Man's array of powers. As it turned out, Yukk!'s "powers" were often the only means by which Mighty Man could escape from the occasional deathtrap and/or take down otherwise impervious weapons or machines.
Strangely enough, despite the extreme ugliness of Yukk!'s face and the effect that it has on almost everyone and everything else around him, both Mighty Man and his alter ego Brandon Brewster have proven to be almost completely immune to negative effects from Yukk!'s face, even at "close range"; greatly repulsed, to the point of screaming, but otherwise unharmed.
Yukk! himself was completely unaware of how ugly he was, and always attributed the effects of removing his doghouse to luck, the weather, or other coincidences, so he usually had to be tricked into removing his doghouse. His powers would often backfire if his doghouse was inadvertently removed by outside forces or accident, imperiling friend and foe alike.
Yukk!'s character is voiced by Frank Welker, years before he won the role as the voice of Megatron in the Transformers cartoon series — alongside his colleague Peter Cullen, who was the voice of Megatron's enemy, Optimus Prime, and who provided the voices of Mighty Man and his alter ego Brandon Brewster in this cartoon series.