nother suggestion is that 'raining cats and dogs' comes from a version of the French word 'catadoupe', meaning waterfall. Again, no evidence. If the phrase were just 'raining cats', or even if there also existed a French word 'dogadoupe', we might be going somewhere with this one. As there isn't, let's pass this by.
There's a similar phrase originating from the North of England - 'raining stair-rods'. No one has gone to the effort of speculating that this is from mythic reports of stairs being carried into the air in storms and falling on gullible peasants. It's just a rather expressive phrase giving a graphic impression of heavy rain - as is 'raining cats and dogs'.