Abyssal Terrors

Abyssal Terrors is a term given to a wide variety of terrifyingly powerful monsters, living or undead, that serves as combat mounts for only the most war-like of the Vampire Lords. Some of these monsters are creatures of Chaos born from the mountains, the resurrected hybrids of rapacious beasts or resurrected monsters that were given life with the use of Necromancy. Others are nightmarish creations of mad science and dark sorcery, bound with shadows and given bodies of writhing blood and flayed skin. The most common of these variants are generally winged creatures.

Most Abyssal Terrors are inevitably borne to war on ragged wings, allowing their Undead masters to strike at the heart of the enemy army. The latter-day Von Carsteins, for their part, were known for their use of a huge wolf-headed variant with slavering jaws and leathery wings. The exposed spinal columns and bony tails of this particular mount oozed with a numbing poison that drew all warmth from those it infected. The creation of these disturbing constructs has more to do with the unholy science of those who follow the dark arts than with any natural process.

The most talented of the Vampires that create them, such as the hidden brotherhood of the Necrarchs, draw a twisted amusement from such blasphemous births.They use parts harvested from a wide variety of monsters, fusing bone with ragged muscles and sinew together. Their vile creation is finally given animus when Morrslieb is at its fullest and a portion of that cyclopean moon's power is invested in the beast as it lurches and twitches upright. With it's birth, the creature is forever bound to the will of his Undead masters.

Source

 * Warhammer Armies: Vampire Counts (8th Edition) -- pg. 51