Demigryph Knight

"They rode their powerfully agile steeds up the sides of the valley, arcing around and down upon the flanks of the Greenskin Horde. There, they waged a cull of such unremitting violence that it broke the deadlock and, ultimately, carried the day"

- Knight of the Vengeful Sun, during the Battle of Ghoul Pass

A Demigryph Knight is one the Imperials most powerful Shock Calvary, and are often tamed from the wilds to be fielded in the heat of battle by one or more Knights, sometimes from a Knightly Order. These massive beast are very powerful, often larger then the biggest warhorse, and many times more powerful and deadly. Not only are the knights riding these beast expert horsemen and swordsmen, the beast they ride are even more fearsome then the rider. Upon conflict a Demigryph will use its iron-hard claws, and its sharp beak to either decapiate or dismember any foe that seeks to harm it, or its rider. Although having a fearsome reputation, Demigryph Knights are rare, as these beast are still fewer in numbers in order to mass-produce for the war's the Empire faces. These beast can only be found in one area in the whole Old World

In the heart of the Reikwald Forest there lurk many foul and fearsome creatures. However, there also dwell many magnificent creatures from ages past of a more noble heritage that can be harnessed for war by the Knightly Orders of the Empire. Only the Knights of the Inner Circle, a cadre of the most honoured and skillful of the Knights of many Knightly Order have the worthiness to be honored to ride these exotic, and frankly ferocious beast of war. Just as having a large reputation as fearsome beast, those warriors riding it however are more often then not the most virtuous and battle-hardened knights of the Empire, for these beast are second to none in terms of loyalty, companionship, and a fearsome repuation on the battlfields in the Empires long proud history.

Source
Warhammer Armies: The Empire 8th Edition pg 45 - 46