The Collaborative International Dictionary of English v.0.48:

Rearmouse \Rear"mouse`\, Reremouse \Rere"mouse`\ (-mous`), n.
   [AS. hr[=e]rem[=u]s; probably fr. hr[=e]ran to agitate, stir
   (akin to G. r["u]hren, Icel. hr[ae]ra) + m[=u]s mouse.]
   (Zool.)
   The leather-winged bat (Vespertilio murinus). [Written also
   reermouse.]
   [1913 Webster] re-arrange

WordNet (r) 3.0 (2006):

Vespertilio murinus
    n 1: common Eurasian bat with white-tipped hairs in its coat
         [syn: frosted bat, Vespertilio murinus]