In the 1500s, houses had thatched roofs. Thick straw, piled high, with no wood underneath. It was the only place for animals to get warm, so all the pets ... dogs, cats and other small animals, mice, rats, bugs lived in the roof.
When it rained it became slippery and sometimes the animals would slip and fall off the roof. Hence the saying, "It's raining cats and dogs."
Warning!
Below is the answer to an age-old question, however, due to the nature of the subject some (who have no sense of humor) may be offended.