Wyvern

Wyverms are cannibalistic predators that are a distant off-shoot of the Dragon race. These creatures have been known to be oppurtunistic hunters, able to eat just about anything from rotting carcasses to poisonous bloated bodies amongst other filthy habits, making they Wyverm one of the most foul-smelling creatures to ever plague the World's Edge Mountains. These creatures have been known to swoop down from their bone-filled lairs high up in the mountains to devastate Empire towns and farms for easy prey. When the Men of the Empire see a Wyverm in the distance, they often go into a panic. For the Greenskins, its the entire opposite.

When they see these creatures circling above, Orcs have been known to greet it with raucous cheers, for the flying creature represents both might and brutality, traits that are highly respected within Orc society. Perhaps the only way for an Orc to ride a Wyverm is to steal a Wyverm egg and hand-rear it to maturity. These beast are the most highly prized mounts within a Greenskin army, where only the most powerful Orc Warbosses or Great Shamans have the ability and unnatural bond to bind these cruel beast to his will. In battle, Wyverms are voracious beast that always hunger for fresh meat, and would sometimes land right in the middle of an enemy unit and feast upon their flesh in the droves while its rider hacks at the enemy that are still left.

Source

 * Warhammer Armies: Orcs and Goblins (8th Edition) -- pg. 63