Several hundred years ago, most homes in Europe had thatched roofs. An extremely hard rain would soak through the thatching and literally wash stuff out of it. Yes, cats, dogs, snakes, bugs, nested in the stuff.
This is also why beds back then had canopies.
Koreans and Taiwanese say it is "raining ropes". Everybody has a euphemism for it.
Because that is where cats and dogs originally came from, clouds.