Morbo is Eunice & Milford, later revealed to be mad scientists Wilhelm & Constance von Stroheim's, deformed handyman in "The Sinister Fate!!".
Morbo is first mentioned when Milford says he's going to play tiddlywinks with Morbo in the lab, only to shut himself in the closet of their home which is actually an entrance to their secret lab.
When they later reveal their plans to swap their brains into Hayley and Francine's bodies, they order Morbo to capture and bind the pair. After Francine is sedated, Constance is distracted while abusing Morbo for touching Hayley's hairallowing Hayley to slip her bonds and knock Constance away from slicing into her mother's skull. Constance tries to send Morbo after Hayley, who reminds him of the beatings he's suffered at her hands. He instead turns on Constance and her husband, allowing the Smiths to escape.