In the Devonshire dialect in England, an airymouse, or aerymouse, is a bat.
There is a nursery rhyme that begins "Airymouse, airymouse, fly over my head," but I can't find the rest of the poem.
It is also the title of a 1967 book by Harald Penrose, about his beloved single seater airplane.
Editor's note: the full rhyme goes thus:
Airymouse, Airymouse, fly over my head,
And you shall have a crust o' bread;
And when I brew and when I bake,
You shall have a piece of my wedding cake.