When the rain is especially heavy, an English person might remark that it’s “raining cats and dogs.” Although the etymology of the expression is unknown, there are documented occasions where animals have literally fallen from the sky during rainfall. Over the last 150 years there have been incidences involving fish, frogs, jellyfish, worms, and spiders.
Although a definitive explanation has yet to be found, the most recent explanation offered is tornados or waterspouts transporting animals to high altitudes before depositing them in a localized area. This theory, however, doesn’t explain why these incidents involve just one kind of animal, instead of a variety of species.
Wikipedia: Raining Animals