Webster's Revised Unabridged Dictionary · 1913

Rearmouse

Rearmouse (-mous`) , noun

[Anglo-Saxon hrēremūs; probably from hrēran to agitate, stir (akin to German ruhren, Icelandic hrara) + mūs mouse.]

(Zoology) The leather-winged bat (Vespertilio murinus).

Also: Reremouse