Piebald rats were enormous sewer rats found in Undertown. They were white with black spots. Piebald rats also knew instinctively when the sun was rising, even down in the sewers, and sang each morning as the first light appeared over the Undertown horizon. Though considered inedible by most Edge inhabitants, there is evidence that they can be eaten.
A frequent rumour spread on the Viaduct Steps of Sanctaphrax was that piebald rats, muglumps, white ravens, or even deceased academics were cooked in the refectory. Objections by people who worked in the refectory were dismissed with the answer that piebald rats could grow to an enormous size and take on all kinds of forms, some with extra lungs, heads, or even wings.