Nightmares

Nightmares are midnight-black horses with flaming eyes and hooves.

Sightings of nightmares are rare. The largest number of them seen in any one place are the keep of Rabindrinath in the Dream Realm, where he keeps a couple in his stables. They are all hostile.

The only sight of one in the waking world are in the Mountains of Freedom, where one is kept as a pet (possibly by the mad sorcerer Lorthondo). The nightmare is described as a 'treasure and daemon in one,' and also as male (indicating that either not all nightmares are mares, or that the writer of that particular letter knows nothing of equine anatomy).

Trivia

 * For some kind of reason, Smith the Horse appears in the Dream Realm as one as well.