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.
It can be noted that Bullhorn can supposedly take on Keefe if he tried to escape as mentioned in "Keefe's Short Story" in Nightfall.
Note: Bullhorn is, presumably accidentally, called Bullseye on page 337 (hardcover) of Nightfall.