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.
-- From E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898. Courtesy of Bartleby.com.
I had a black mouse once. I can't remember her name, which is awful, but I remember how friendly and engaging a pet she was. She liked to be petted with a pinky finger, and liked to nest in my scarf for a few minutes if I was wearing one. Her tail was as elegant as a Chinese calligrapher's line. When she died I buried her in a teabag box full of dried rose petals.