As an animator, Friz Freleng enjoyed creating new adversaries for Warners' star Bugs Bunny, since he felt that Bugs' other nemeses, such as Beaky Buzzard and Elmer Fudd (who actually appeared in many more Freleng shorts than is commonly realized), were too stupid to give the rabbit any real challenge. Considered revolutionary for almost all of the late 1940s though he might have been, Freleng's own Yosemite Sam had not yet been proven capable of fully fulfilling his creator's intentions. Freleng introduced two of these more formidable opponents as a pair of gangsters in the 1946 film Racketeer Rabbit. In the film, Bugs decides to find himself a new home, but the one he chooses is unfortunately occupied by a duo of bank robbers. The characters here are called "Rocky" (drawn like movie gangster Edward G. Robinson) and "Hugo" (a caricatured Peter Lorre). Both gangsters are performed by the Warner studio's longtime chief voice actor, Mel Blanc.
Freleng liked the mobster idea, and he used the concept again in the 1950 short Golden Yeggs. This time it's Porky Pig and Daffy Duck who run afoul of the Mob, only this time Rocky has not only one sidekick, but an entire gang. Freleng also redesigned Rocky for this short, making him a more generalized caricature of the "tough guy" gangster rather than Robinson in particular. Freleng used several of the same techniques that would make Sam, his other Bugs villain, such a humorous character: Despite Rocky's tough-guy demeanor, everlasting cigar (or cigarette) and foppish gangster dress, he really is little more than a dwarf in a much-too-large hat.
In 1953's Catty Cornered, Freleng set the Mob against another of his comic duos, Sylvester the Cat and Tweety Bird. Gang leader Rocky, this time aided and abetted by a hulking simpleton named "Nick", kidnaps Tweety Bird, and when Sylvester's bumbling predations accidentally free the bird, the poor puss is hailed as a hero. The duo reappear in 1954's Bugs and Thugs, this time in the form Freleng would keep them for the rest of their run. "Nick" is here rechristened "Mugsy", and though his over-muscled body stays mostly the same, his hair is gone, and his facial expressions are decidedly less intelligent. Before the Warner studio closed for good in January 1965, Rocky and Mugsy would appear in two more Freleng cartoons: Bugsy and Mugsy (1957) and The Unmentionables (1963). Mugsy also appears without his boss in a cameo as one of Napoleon Bonaparte's guards in the 1956 Freleng short Napoleon Bunny-Part.
Rocky and Mugsy have also appeared in various Looney Tunes-related merchandise. They are semi-regular characters in Looney Tunes comic books, for example. They also play the villains in the 2002 Xbox video game Loons: The Fight for Fame, a vs. fighting game in which the no-good gangsters attempt to run a film studio into the ground so they can buy up the stock for next to nothing. Also in Bugs Bunny Lost in Time the pair are bosses of the 1930s era. They also appeared in episodes of The Sylvester and Tweety Mysteries and Duck Dodgers.