Reavers are an oddity to Sera. They are a flying animal that most resembles a squid. Their limbs are clawed tentacles which allow them to latch onto surfaces and also have a few small claws on their underbelly that also allow them to latch onto surfaces. But there are no signs, whatsoever, of any organs that allow Reavers to fly. Like most creatures from the Hollow, Reavers are controlled via Locust technology, making them completely obedient to the pilot. Only two passengers can fit on a Reaver, a pilot, that can accurately fire rockets (but once killed the Reaver fires rockets left, right and centre) and a gunner, to mop up any straddlers.
In Gears of War, they don't make a big appearance, but they're most notable for their encounters on the train during the final chapter. They also back up RAAM during the final boss fight.
In Gears of War 2, they can be seen almost immediately (i.e. in the first act). They lead a main offensive against the assault derricks that are taking the COG to an area where Hollow access is easy. Another encounter is when Baird is fixing the Centaur tank, multiple Reavers attack, acting like mini-boss fights. The Hydra, a special Reaver piloted by Skorge, is later killed by Marcus, Dom, Baird and Cole after capturing their own Reavers, showing that Reavers only show true allegiance to whoever is controlling them.
In Gears of War 3 they primarily appear during the Gas Barge journey to Avenguard and can also appear as a boss for the boss wave of Horde