OK...that would hurt a lot if I or anyone went outside in this weather and got pelted with a bunch of cats and dogs especially if they were the size of a Lab or Retriever. I guess the small ones would be the calm before the storm and get ready for the big guys. So, I asked myself what does it mean and this is what I came up with.
"The most common one says that in olden times, homes had thatched roofs in which domestic animals such as cats and dogs would like to hide. In heavy rain, the animals would either be washed out of the thatch, or rapidly abandon it for better shelter, so it would seem to be raining cats and dogs." (google search)
So I guess they were being hit left and right and from on top by cats and dogs way back when!