Michigan J. Frog was born on New Years' Eve, 1955 with the debut of "One Froggy Evening." The Looney Tunes cartoon short told the story of a man who while working one day discovered a box which contained therein a frog. A frog that could sing and dance. The ambitious fellow tried to cash in on his newfound spectacle, only to find out that the frog only performed for him and him alone. To others, he was merely a frog like all the others. The gag grew to ever-increasing heights of fancy, until finally the frog was released back into the wild, where years later, he is picked up by another hard-working man, who quickly hatches another scheme ... ad infinitum.
The frog (named Michigan because of his trademark song "Michigan Rag", penned by Looney Tunes music director Milt Franklyn) was one of the few WB characters not voiced by the ubiquitous Mel Blanc; instead, a local radio talent named Bill Roberts was called in to provide the frog's distinctive vaudeville baritone. The reason for this is that Mel went on vacation in December, and thus the studio used this time to make its voiceless cartoons and other featurettes. Regardless, Roberts did the voice, and slipped back into obscurity, while Michigan remained.
"One Froggy Evening" was a huge hit (it ranked #2 on Cartoon Network's recent list of the 50 Greatest Cartoons of All Time), and even spawned a prequel, "Another Froggy Evening", tracing Michigan's path through history, leaving numerous greedy suitors in his wake.
Michigan languished for the next 40 years after his debut, appearing in one of the Bugs Bunny clip movies but generally avoiding the spotlight, until finally on January 11, 1995, when he was reborn as the mascot of the WB Network, a new network begun by Warner Brothers. Unlike the original frog, the new mascot did not sing (though he did dance), and he was obviously not too modest to perform for the camera this time. Michigan has continued to be the mascot to this day, and presumably will do the job for years to come.
It's amazing how much mileage you can get out of a singing frog.