Lewis & Short

† Hȳdra, ae, f., = Ὕδρα [kindred with Sanscr. udri; Ang.-Sax. oter, otor; Engl. and Germ. Otter; cf. also the Gr. ἔνυδρις],

  1. I. the water-serpent killed by Hercules near the Lernean Lake, the Hydra, with seven heads; as fast as one of them was cut off two sprang up in its stead; it is also called Echidna: Lernaea pestis, Hydra, Lucr. 5, 27; Ov. M. 9, 192; Hor. C. 4, 4, 61; id. Ep. 2, 1, 10; Hyg. Fab. 30; 34; 151. As identified with Echidna, the mother of Cerberus, Cic. poët. Tusc. 2, 9, 22.
    Prov.: vide ne in istis duobus generibus hydra tibi sit et pellis, Hercules autem et alia opera majora, in illis rebus, quas praetermittis, relinquantur, i. e. the easiest, the least important, Cic. de Or. 2, 17, 71.
    1. B. Deriv. Hȳdraeus, a, um, adj., of or belonging to the Hydra: germen, Mart. Cap. 7, 237.
  2. II. Transf.
    1. A. The constellation of the Water-snake, also called Anguis, Cic. Arat. 214 (also id. N. D. 2, 44, 114); Hyg. Astr. 2, 40; 3, 39.
    2. B. Acc. to Verg., a hydra with fifth heads, that keeps watch at the gates of the Lower World, Verg. A. 6, 576.