"Why do these black horses endure such riders?"asked Frodo.
"These horses are born and bred to the service of Dark Lord of Mordor.Not all his servants and chattels are wraiths! There are orcs and trolls, there are wargs and werewolves; and there have been and still are many. Men, warriors, and kings, that walk alive under the Sun, and yet are under his sway. And their number is growing daily." said Gandalf.