Sources : Hydra

Isidore of Seville [7th century CE] (Etymologies, Book 12, 3:): The hydra is a dragon with many heads; this kind of dragon was in the Lernean swamp in the province of Arcadia. It is called excetra in Latin, because when one head is cut off, three (caesus tria) more grow back. But this is only a story; for it happens that Hydra was a place that spewed out floods that devastated the neighboring city. If one outlet for the water were closed up in this Hydra, many others would burst forth. When Hercules saw this he dried up the place itself and thus closed up the path of the floods. Hydra is named from water. - [Barney, Lewis, et. al. translation]