A Mummerset black was a particularly vicious type of boar. Nyssa once had to go on the run from one of these rampaging beasts of Mummerset, which she only escaped by throwing her shoes away. Since the beast had caught the scent of the soil on them, it was easy enough to send the beasts away by discarding the shoes in the opposite direction.
Maud the Withered claimed that they were omnivores and even ate humans. (AUDIO: Castle of Fear)
Behind the scenes
Since Mummerset itself is a fiction, so are these animals.