An Etymological Dictionary of the German Language/Fledermaus
Fledermaus, f., ‘bat,’ from the equiv. MidHG. vlëdermûs, OHG. flëdarmûs, f.; corresponds to Du. vledermuis; E. flittermouse does not occur in AS., and may be due to the influence of MidEurop. Teutonic. That the animal was thought to be a mouse is shown by AS. hreape-, hrêremûs; the E. term bat, MidE. backe, Dan. aftenbakke (aften, ‘evening’), is unique. Fledermaus, lit. ‘fluttering mouse,’ from OHG. flëdarôn, MidHG. vlëdern, ‘to flutter.’