Sources : Hydrus

Isidore of Seville [7th century CE] (Etymologies, Book 12, 2:36): The enhydros is a small animal; it gets its name because it lives in water, specifically the Nile river. If it finds a sleeping crocodile it first rolls in mud, then crawls into the crocodile's mouth; after eating all of the crocodile's inner parts it comes out of the beast, killing it.