Rabid Possum is a creature that appeared at the very end of the episode, "The Prince Who Wanted Everything." At first Lumpy Space Princess and the Ice King thought it was going to be Lumpy Space Prince brought to life out of the bushes, the the possum appeared instead. The possum then climbed into Ice King's cloak, ending the episode.
It appears to be a possum with red eyes and a foaming mouth.
Opossums are actually quite capable of fending off rabies, as the internal temperature of an opossum is too cold for rabies to infect properly.