The Horrible entity known to the Elves of Cuivienen as The Rider or The Hunter was an evil Maia and the greatest of the Hunters of Morgoth. Morgoth personally entrusted him to capture many elves in hopes to twist them into the unfathomable abominations that they would eventually become.
So powerful and terrible was this horseman, that many speculated that the horseman was actually Morgoth himself, others however believed it was Sauron, Fankil or Tredeinos, one of Morgoths demonic Chieftains.