Nightmare

Nightmares range from dark indigo to black, with manes, tails, eyes, and hooves that glow like moonlight at night or in the dark. During the day, these tend to be cream colored or off white, often with pearl highlights. Nightmares are creatures of the night and the dream realm, but contrary to popular belief, they often opt to protect others from bad dreams. Because they're blind during the day, they tend to avoid contact with the waking world.
These elusive creatures reproduce by breeding, and almost always produce fillies. The rare stallion born to a nightmare remains part of the Dreamscape, and can only manifest in a physical form once a season. If he is lucky, during that night, he may find a nightmare and produce an offspring. In the Dreamscape, they appear as ethereal white horses, typically only to children, to assist them with their dreams if called upon to do so. They are the guardians and sentinels of the realm of dreams, fiercely protective and devoted to its preservation.

by JustBirch