Bill the Cat is a character in Berke Breathed's comic strips Bloom County and Outland. Physically, he looks rather like Garfield (of comic strip "Garfield", by Jim Davis) would look, if skinned, taxidermized, all stuffing removed, and the remaining skin filled about halfway with suet. This is purely intentional, as I will explain in a moment.
According to Breathed himself (in the back of one of the Outland collections, I believe; I don't remember which one), Bill the Cat was created as a jab at the cute, fuzzy animals prevalent in other comics; Breathed wanted a character with "zero, or maybe even minus, merchandising appeal". To this end, Bill has essentially all of the flaws of a real animal, exaggerated for comic value: he is ugly (albeit in a somewhat lovable way), makes rather disgusting noises (notably "ack" and "thpppth"), and produces a variety of hairballs and secretions (he has been seen on the litter box more than once, often reading a newspaper). This effort proved rather ironic, however, as possibly only Opus the penguin now surpasses Bill in terms of total merchandising dollars in Breathed's pocket.
Like Opus, Bill was originally a throwaway joke, but ended up being a recurring character under the influence of fan pressure, dumb luck, or both. Given the popularity of both characters, this may begin to indicate a pattern.
Though he was initially as animalistic as most members of his species, Bill has since demonstrated surprising intelligence as mute animal (as opposed to talking animal) characters go. Among other misadventures, he has been killed in a car crash, run for president, and been cloned from tongue cells. He has also been seen dating numerous female celebrities, including Princess Diana. | 
