Pliny the Elder [1st century CE] (Natural History, Book 29, 22): The most beautiful snake in the world is the kind called hydri, that is amphibious, no other snake being more venomous. Its liver when preserved does good to those who have been bitten. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 2:36;): The enhydros is a small beast so named because it dwells in water, particularly in the Nile. If these beasts discover a sleeping crocodile, first they roll themselves in the mud and then enter the crocodile’s stomach through its mouth, and eat everything inside it, so that the crocodile dies. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Serpents 8.20): Idros is a serpent in the river Nile. When it sees a crocodile sleeping with its mouth open on the banks of the Nile, it rolls itself in muddy slime, so that he could more easily get into the crocodile's mouth. Then the disturbed crocodile, swallows the slippery serpent, which soon emerges alive from the crocodile's torn entrails. As Pliny writes, the ydros is the most beautiful serpent in the world. The liver of this serpent is used as a remedy for those who are bitten. When people are stung by this snake, they are stupefied, which they call the boa disease, because it is cured by cow [bos] dung. - [Badke translation/paraphrase]

Guillaume le Clerc [ca. 1210 CE] (Bestiaire, Chapter 21): There is a kind of serpent / Which has its abode in water. / Hydrus is its name, it is very wise, / For it knows full well how to do hurt / To the crocodile which it hates; / It knows how to entrap it cunningly. / I shall tell you first clearly / How this creature entraps it so cleverly. / The other beast of which I told you, / Which lives always in the water, / Hates the crocodile with deadly hate, / And it the other and no mistake. / Much is it filled with hatred, / But the other is much more cunning. / When on land it sees it sleeping / And when sleeping to open its jaws, / In mud and slime it bathes, / And rolls in it and smears itself / For to be more slippery. / Then it goes straight for that devil, / Down its throat it darts and is / Swallowed by it into its belly. / And there is no great time passed / Before it bursts it open and tears / All the entrails of its belly / And its bowels and intestines. / It seeks a way out quickly, / And so gets out quite safely. / And the other dies; for die it must, / For of its wounds it cannot recover. - [Druce translation]