Bullhorn is Elwin’s pet banshee. Elwin keeps him in the Healing Center at Foxfire. Bullhorn is described as looking like a demented ferret with purple eyes and gray. Banshees can sense when someone's life is in danger and will scream or sleep on those near death. Bullhorn has laid on Sophie twice when she was near death after having an allergic reaction and also almost fading away.
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 (assumedly) accidentally called Bullseye on page 337 (hardcover) of Nightfall.