Swamp Hag

A cackling, warty witch who turns up in the swamp muck of New New York with unsettling regularity, stirring pots, casting glances, and cackling at nothing in particular. She has no discernible role in whatever plot is unfolding around her — she simply appears, does something witchy, and vanishes, leaving everyone else to carry on as if nothing happened.

She's a pure non-sequitur gag, and the show never explains her, which is the joke. Tress MacNeille gives her a gleeful, rasping cackle that makes each unannounced appearance land harder than it should. The less sense she makes, the funnier the reveal that Futurama expects you not to ask.