E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Mouse.
The soul or spirit was often supposed in olden times to assume a zoömorphic form, and to make its way at death through the mouth of man in a visible form, sometimes as a pigeon, sometimes as a mouse or rat. A red mouse indicated a pure soul; a black mouse, a soul blackened by pollution; a pigeon or dove, a saintly soul.
1
Exorcists used to drive out evil spirits from the human body, and Harsnet gives several instances of such expulsions in his Popular Impositions (1604).
2
No doubt pigeons were at one time trained to represent the departing soul, and also to represent the Holy Ghost.