Reavers are flying creatures used by the Locust for air combat. They appear in Act I but aren't actually fought until Act V.
A Torque arrow can bring down one of the creatures in a single shot. Also, sniping the pilot will kill the creature instantly. The Boomshot works well if you have it, if not, the creatures can be brought down with a Lancer but it would be best to run if that is your only option.
 Gears of War 2
These are fought multiple times through out the campaign. There are usually 1 or 2 Drones on top of it. One of them will be controlling the direction of where the Reaver flies and the other usually uses a Hammerburst. When a Reaver lands on the ground to attack you, it will either shoot a rocket-like object at you, or if it gets close enough, it will hit you with its claws, either downing you or damaging you severely. Easy ways of killing these are by shooting it with any weapon from a distance, or go under it where it cannot hit you, and shotgun it to death. There's another relative specie of the Reaver called the Hydra.