These horses are psychically gifted maow that coexist with humans. They have their own nomadic culture living in small herds in the mountains and forests where they have an unearthly grace and speed, uncannily dodging the trees and roots of the forest.
They can telepathically communicate emotions to each other and other psy-gifted creatures. Creatures without a psychic ability are able to summon/call to a Shadow Horse via a specially enchanted gem that is particular to the horse.
Despite being nomadic, they are feircely protective of the forests, and long have related to the sylvan culture. A few rangers have befriended a shadow horse to the extent that the horse--most often a male--will leave its herd to stay behind with the ranger, helping to protect the territory and "serving" as the ranger's mount. They prefer to stay within the trees, however, and will only come into the open at night.
Examples of such horses include the black stallion that serves Laurel with the ranger Giacomo and the black mare that assists Zellistae.