The name Night-Mare is a misnomer, though not entirely without foundation. During the founding years of the Realm Aterrizar, the mythozoological species of Equus Vigilia was brought to the very edge of extinction.
The species was dwindled down to a herd of 13 mares. Now, in ancient times, this breed of animal wandered in and out of the realms of unconscious dreaming, delivering dreams and visions, prophecies and nightmares to whom they were meant to. But some were disconcerted by this and sought out the herds of the horses, and brought battle to them. While the petty humans were initially poor at this task, they adapted quickly, and gained the upper hand. Within a few generations, the Night-Watch horses were all but destroyed. It was during this bleak time that several sought out patrons among the unholy and the damned as they felt betrayed by the forces of light and the Dreaming.
They became fell and black, their hooves strike stone like steel, showering the cold earth in silent sprays of sparks. Flame and the scent of brimstone followed them as they thundered up out of the various hells to bring their dark gifts to a world that had scorned them, and declared itself too serious, and unneeding of petty concerns of daydreams and flights of fancy. During this time, the Unicorn was driven into hiding, and pity the pegasi whom were slain to the bitter last for the gift of their wings. Curses upon that bloodline that spilled the heartsblood of the last.
It was during the sixth turning of the wheel that the few nightmares made their way to Aterrizar, escaping their infernal stable-masters and demonic groomsmen. They had followed the bright path and found a realm that sang of the old ways, and they were unharmed by the light of the blowing sun. They laughed and froliced in the seas of grass, and were for a time contented.
But soon, there was discontent, they were still but 13, and the urging of the moon was in their loins, but there were no stallions left in the whole of creation. The answer came in the form of a thickly muscled, and scarred unicorn stallion.
The first generation of Equus Vigilia, more correctly E. Vigilia-Monoceros displayed the black coloration of the night-watch, but also had the long leonine tail and horn of a unicorn. This hybrid became less and less noticable as the E. Vigilia continued to breed back into their own herd, as well as crossing over into the few unicorn mares who were willing to breed.
From this came the restored bloodline of the Nightwatch horses, which are still refered to as Nightmares despite the presence of both genders, and the ill-tempered breed of Black Unicorn, E. Monoceros-Noctus.
The 13 foundation mares still exist, having been blessed, or cursed with immortality. As such they are quite intelligent and have garnered powers well outside the norm for a magical species. Each has the normal nightmare powers over dreamcrafting, immunities to fire and such. Each also has the power to appear a human being, limited spell-casting powers, and the ability to hide from prying eyes.
The Nightwatch breed is tied to the cycle of the moon, becoming the most agressive during the full-moon, and all but vanished during the darkness of the new moon.