Bullhorn is Elwin's pet banshee. Elwin keeps him in the Healing Center at Foxfire. He is described as looking like a demented ferret with beady purple and grey eyes. A banshee can sense when someone's life is in danger and will scream or sleep on those near death. It has been said that when a banshee lays near you, it is a much worse sign then if it were screaming. Bullhorn has laid near Sophie three times; when she was near death after having an allergic reaction to limbium, after almost fading away, and after Umber attacked her with shadowflux.
Note: Bullhorn is, presumably accidentally, called Bullseye on page 337 (hardcover) of Nightfall.