An Etymological Dictionary of the German Language/Annotated/Otter
Otter, feminine, from the equivalent Middle High German otter, Old High German ottar, masculine, ‘otter’; corresponding to Dutch otter, Anglo-Saxon otor, English otter, Old Icelandic otr, ‘otter’; Gothic *utrs is by chance not recorded. Gothic tr remains unaffected by the High German permutation; see bitter, lauter, treu, zittern. The term udrá- is applied in several Aryan languages to aquatic animals; *udra- belongs to the same root as Greek ὕδωρ, ‘water,’ ἄνυδρος, ‘waterless, dry,’ Sanscrit udan, ‘water,’ ánudra-s, ‘waterless, dry,’ hence Modern High German Otter is etymologically connected with Wasser. Compare Greek ὕδρα, ὕδρος, ‘water-snake,’ Lithuanian údra, ‘otter,’ Old Slovenian vydra, ‘otter,’ Sanscrit udra, masculine, ‘otter.’ —
Otter, feminine (thus in Luther), for ‘adder,’ is East Middle German; compare Dutch and Low German adder, English adder (also Suabian ā̊dr).