Sources : Hydros

Isidore of Seville [7th century CE] (Etymologies, Book 12, 4:22): The hydros is an aquatic snake; people struck by it become swollen. Some people call the ill effects of this boa, because it is remedied with cow [bos] dung. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Serpents 8.21): Ydros is a river serpent that does not live in marshes, as the Liber Kyrannidarum says. It raises its chest above the waters. It carries a stone in its head, which it vomits out when flayed alive or fumigated. It was also seen to have vomited a stone, sworn by the living God, the Lord of heaven and Earth. I myself proved the power of this stone: I applied the stone to a certain woman with dropsy [fluid retention disease] and every day I measured her belly; it went down by three inches, until it returned to its proper place. I therefore removed the stone, lest her natural moisture should dry up too much. The same applies to all rheumatism, to every flow of the eyes, and to every rheumatic limb. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.9): And some be water Adders, and dwell in brimmes of waters, as the Serpent Enidris, that is a water adder, and who so is smitten at that Adder, hée swelleth into dropsie. And many men call it Bova, for the durt of an Oxe is remedy therefore, as Isido[re]. sayth, lib. 8. - [Batman]