The Fellbeasts, also referred to as the Hell-Hawk or Nazgûl-Birds are gigantic flying creatures which the Nazgûl use as transportation. They are a possible enemy in Middle-earth: Shadow of Mordor.
Tolkien described the fellbeasts as follows: "...It was a winged creature: if bird, then greater than all other birds, and it was naked, neither quill nor feather did it bear, and its vast pinions wew as webs of hide between horned; and it stank."
Alongside this, their birth is unknown, but it is recounted that they were bred along the Northern mountains of Gorgoroth, as Tolkien writes: "A creature of an older world maybe it was, whose kind, lingering in forgotten mountains cold beneath the Moon, outstayed their day, and in hideous eyrie bred this last untimely brood, apt to evil. And the Dark Lord took it, and nursed it with fell meats, until it grew beyond the measure of all other things that fly; and he gave it to his servant to be his steed".