The actual saying is "Its raining cats and dogs". Guess thats because cats and dogs are so plentiful.

The most common: Houses had thatched roofs, thick straw pile high. It was the only place for animals to get warm, so all the pets, dogs, cats and other small aminals, mice rats, bugs, lived in the roof. When it rained, it became slippery and sometimes the animals would slip and fall off the roof. This is also the reason why four poster beds developed. The idea of the 'roof' was to keep from nasty things falling into your open mouth when asleep! The side curtains kept out the drafts. there are many other derivations to this phrase, one also being: It has also been suggested that at one time the streets of British towns were so poorly constructed that many cats and dogs would drown whenever there was a storm; people seeing the corpses floating by would think they had fallen from the sky, like the proverbial rains of frogs. ALSO It seems that cats were at one time thought to have influence over storms, especially by sailors, and that dogs were symbols of storms, often accompanying images and descriptions of the Norse storm god Odin. So when some particularly violent tempest appeared, people suggested it was caused by cats (bringing the rain) and dogs (the wind).

I was curious about this myself after seeing your question, so I looked it up on Wikipedia. It didn't give me the answer I was seeking, but instead redirected me to "raining animals." It then proceeded to tell me about numerous instances where it "rained" various animals from the sky. These occurrences were in the UK and the USA, and it seems the most common animals for it to rain are fish, frogs and birds (in that order). The following quote comes from the article: "Sometimes the animals survive the fall, especially fish, suggesting a small time gap between the extraction and the actual drop. Several witnesses of raining frogs describe the animals as startled, though healthy, and exhibiting relatively normal behavior shortly after the event. In some incidents, however, the animals are frozen to death or even completely enclosed in blocks of ice. These occurrences may be evidence for the transport of the victims to high altitudes, where the temperature is below zero, and they show how powerful meteorological forces can be. The potential violence of this phenomenon is shown by examples where the product of the rain is not intact animals, but shredded body pieces." In addition to raining animals, the page provides a link to "Exploding Animals" which I was a little too afraid to follow!