There is considerable confusion in applying the name hydrus and its variations to beasts. The root of the word itself refers to water, and this led to several beasts, mostly serpents, being so labeled. Isidore of Seville lists the hydros, a water snake that causes those bitten to swell up, the cure for which is the dung of an ox. The hydrus was and is also sometimes confused with the Hydra of the Hercules legend, with some texts saying that it was a many-headed water dragon, living in the swamp of Lerna, that could grow new heads. This comes from the fact that both words were just ancient greek names for giant water serpents.