Dane Whitman, the Black Knight, used his uncles scientific methods and technology to mutate a normal white horse into a winged steed, whom he named Aragorn. The horse served at Black Knight's mounts until Black Knight was turned into stone by the Enchantress, at which point he was placed in the care of Valkyrie. And when Black Knight was returned to normal and decided to spend time in the 12th century, Aragorn became Valkyrie's horse permanently.
Another winged horse used by the Vatican Black Knight, also named Aragorn was killed by Alyosha Kravinoff.
Aragorn can fly by means of various adaptations given him through artificial mutation. The most prominent of these are his wing, which span eighteen and a half feet from wing tip to wingtip. The wings are fully feathered, like a bird's.
Aragorn's entire anatomy has been adapted to flying. His bones are hollower than a normal horse's, making him weigh less than a normal horse of his size would. His body is virtually devoid of fat, possessing greater proportionate muscle mass than a normal horse. His eyes are specially adapted to be able to withstand high-speed winds, which would hurt the average equine eye. He possesses special membranes in his respiratory system enabling him to extract oxygen from air at high velocity of at high altitudes.
He also has some magic properties.