Who is the Horned One in the novel, The Mists of Avalon?

Arthur, Morgaine's brother (half-brother), is the Horned One in the novel.

“Your voice,” he whispered, “and what you just did—why do I seem to know you? Is it because you are the Goddess, and in her all women are the same? No—” He stiffened, raised himself, took her face between his hands. In the growing light she saw the boyish features strengthened into the lines of a man. Still only half aware of why she seemed to know him, she heard his hoarse cry. “Morgaine! You are Morgaine! Morgaine, my sister! Ah, God, Mary Virgin, what have we done?”