And then there's the idea that the phrase came from Norse mythology because people used to believe that cats were in charge of the rain, and dogs were in charge of the wind. So when you put rain and wind (cats and dogs) together, you got a really big storm.
|Raining cats and dogs and pitchforks!|
So what it seems like to me is that a lot of these phrases are trying to say that the rain is coming down so hard that when it hits you, it hurts, like a chair leg hitting you, which is what they say in Greek. Maybe having cats and dogs hit you is not quite as painful as having axes hit you, which is what happens in Serbia, but I still wouldn't want to be standing around when all those animals were falling out of the sky!