The Flittermouse isn’t a mouse, for it has wings, and mice don’t.
But it does fly rather swiftly and dartingly, so it can be said to flitter, and being a mammal with a body not unlike that of a mouse in size, and sometimes in color, it is not surprising that this creature was known to our ancestors as a flittermouse, after the German Fledermaus or the Dutch vledermuis.
But the older and better-known English name was back, a name now lost in English dialects, though remaining as bawkie-bird in Scotland.
Today we know this flying mammal as a bat.