As I suspected, Bruce Tinsley, the rightwing hack who scrawls the wretched Mallard Fillmore comic strip, based his Obama caricature on the one he used to do of John Kerry. It took a bit of doing to find one of Tinsley’s old grotesque caricatures of Kerry - they seem to have vanished from the Internet - but below is one example:
And now compare it to his current drawings of President Obama:
Note the similarities - the grossly elongated chin that sticks off the page, the bushy eyebrows, the buckteeth grin, and so forth.
It looks like Tinsley took one of his Kerry drawings and simply replaced the hair, narrowed the chin a bit and then added big “Dumbo” ears to jut out and flap around.