rearmouse
/ˈrɪəmaʊs/Now rare (archaic and English regional (south-western and midlands ) and Welsh English in later use, or with reference to earlier texts)
noun
A bat.
Origin
Old English; earliest use found in Glosses to Proverbs of Solomon. Apparently from a first element of uncertain origin + mouse, perhaps as alteration of Old English hrēaðemūs.