During the war, Hydras were created by the Locust Scientist Ukkon by mutating Reavers.
Assassination of Ukkon
When confronted by Sergeant Gabriel Diaz and his forces, Ukkon revealed his newest creation, the Hydra and used it to attack them. However, they were able to slay the creature and kill Ukkon.
After the defeat of the Locust Horde during the Lightmass Offensive, one Hydra along with its rider escaped the destruction of the Outer Hollows.
Operation: Hollow Storm
After the defeat of the Locust Horde, 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 and 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.
The Hydra in Greek mythology is a serpentine creature with multiple heads, which would grow two more heads if one was cut off.
The Hydra was called the 'Uber Reaver' in its concept art.
It was referred to as the "Rhino Reaver" by fans up until its official naming and reappearance in Gears of War 2.
In the Gears of War: Judgment Guide book, there is concept artwork that shows the Hydra flying over a burning Halvo Bay. It is possible that the Hydra or another of its species was meant to appear in the game but got cut during development.