The Hydra (also called the Rhino Reaver or Uber-Reaver) was a flying Locust mount similar to the Reaver. Only one Hydra creature appeared during Gears of War 2 and served as the personal mount of the Kantus High Priest Skorge.
Biologically speaking, the Hydra isn't much different from the standard Reaver and is likely part of the same species, perhaps the Reaver equivalent of an alpha male. Notable physical differences include a large pair of mandibles on either side of the lower jaw and thicker tentacles. A large horn also protrudes from the creature's head.
After the Horde's defeat following the Lightmass Offensive, Skorge, the high priest of the Kantus monks, used the awakened Riftworm to destroy Sera's cities on the surface. On his personal Hydra, Skorge led the Riftworm to Tollen and Montevado, destroying the cities. During Operation: Hollow Storm, Skorge, atop his Hydra, led the Riftworm to Ilima, sinking it too and it was first seen by Delta Squad before it led the Riftworm to Ilima. After the death of the Riftworm and the attack on the Queen’s palace, Skorge dueled against Dominic Santiago and Marcus Fenix, however his staff was broken and he was forced to retreat atop his Hydra. Skorge attacked the retreating Gears, following Delta-One who were on Reavers to the surface. The Hydra tried to bite the Reavers, while Skorge tried to shoot them with a Troika until they got stuck in a small exit, though they eventually managed to get out and continue their pursuit. The Hydra and Skorge chased Delta until Delta used the Reavers' machine guns and rocket launchers to destroy the Hydra , severing its claws and blowing up its mouth, which caused Skorge to crash to the ground, killing him.