Mr. Poopy is Eunice & Milford's cat.
When the pair reveal themselves to be wanted mad scientists, their original plan is to place their brains inside of Hayley and Jeff's bodies in "The Sinister Fate!!".
But when their plans are interrupted by the timely visit of Francine and Stan, they slip Stan a sleeping pill and decide to use Hayley and Francine instead for themselves, and give their cat Mr. Poopy's brain to Jeff's body. After convincing Eunice and Milford's deformed handyman Morbo to help the family escape, Jeff scratches his ear with his leg and starts to purr as they head out the door.