Yoda

Yoda was one of the many historic characters that vanished somehow.

He was added to get more people into the strange hole that would take you to Willis World, but he vanished on the same night as all the others. He wasn't seen again, even after Snivy saved the day and Penguin was back to normal. Maybe some day there will be some way to get him back. Most people believe that historic characters vanished, but most think Penguin kidnapped him, and other historic characters. See The Axolotl Experiment for detail.