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Hydrus

"fabulous water serpent," 1660s, from Latin Hydrus, from Greek hydros "water-snake" (see hydra). The constellation (attested by 1670s in English) was one of the 11 added to Ptolemy's list in the 1610s by Flemish cartographer Petrus Plancius (1552-1622) after Europeans began to explore the Southern Hemisphere.