Red Bull
The Red Bull is the story's secondary antagonist. He is the most loyal servant to King Haggard. Under the latter's instructions, the Red Bull had covered up the remaining Unicorns' hoof marks.
Based on a painting Peter S. Beagle found with Unicorns battling Bulls (one of them was Red).
The roars are similar to the Fell Beast on Return of the King (from the same creators).