Hydras are snakey creatures with many heads and many eyes. If hydra's head is cut off more will grow back. One species of hydra has about ten heads (or eleven heads), and twenty eyes (twenty-one or twenty-two eyes).
A knight once took an eye from a snakey hydra. But she was not too happy about it. The Three-headed Dragon is similar to a hydra.
Behind the scenes[]
The Two-Headed Dragon maybe a species of hydra. There was originally going to be a hydra in KQ8, but the seaside level was cut from the game.