Adult Dib made a cameo appearance in "Attack of the Saucer Morons", and has only appeared in one other episode: "Mysterious Mysteries". He only had one line throughout the entire series - in "Attack of the Saucer Morons", after having his head bitten down on by the pig, he said, "The pig accepts me!".
He was seen again in "Mysterious Mysteries", communing with the Blotch with the rest of the Children of the Bright and Shining Saucer.
He is voiced by Rosearik Rikki Simons, though it's hard to tell, because his voice was muffled at the time. It's also unknown as to whether Jhonen Vasquez intended for him to be related to Dib or not.