There is a curious section in Aradia: Gospel of the Witches by Charles Leland that describes the goddess Diana’s incarnation in human form and her marvelous spellcasting to impress the witches. The passage says that “she declared that she would darken the heavens and turn all the stars into mice.” Diana duly accomplishes this feat, the heavens rain with mice, and Diana is crowned Queen of the Witches.
So why does Diana make it rain mice, of all things, to impress the witches? The answer lies in the dual roles of Apollo, as god of light and god of mice. Possibly these roles became syncretized with Apollo as he absorbed many other gods, but at any rate Diana was turning the stars (light) into another of their forms: mice. This is why Diana once chose to make it rain mice.
Ever wonder about the phrase “It’s raining cats and dogs”? Most people recognize this as a description of a fierce rain shower, but have you ever seen cats and dogs pouring down from the sky?
The folklore behind the saying has two parts. A lightning strike in European folklore (and in some other places, such as the Philippines) is associated with a dog bite. That makes sense: electricity has a bite to it, so being struck by lightning probably would feel like a dog bite.
What about the cats? There is also a longstanding belief in folklore that cats cause it to rain by vigorously washing their faces.
So, raining cats and dogs refers to a type of storm where there is torrential rain (caused by the cats) and lightning (biting dogs).