The bat is a winged mouse-like beast; it’s also called a “reremouse”, “flittermouse” or “chausoriz”. 12th-Century bestiaries treated it as a bird, but with membranous wings rather than feathered; medieval thought considered the bat an embodiment of darkness (and dark emotions such as melancholy). The bat is found as an heraldic charge in the arms of Burnynghill, c.1410 [DBA1 289]. It is displayed and guardant by default; indeed, so common was that posture for the bat that using any other posture (including inverted) is deemed a step from period practice.
Elynor O’Brian bears: Argent, three bats sable.
Thomas the Diaemus bears: Vairy vert and argent, a bat displayed sable.
Antoine d’Aubry bears: Quarterly ermine and gules, two reremice ermine.