It is unknown where the English phrase “it’s raining cats and dogs” comes from. But there’s one likely etymology that it is a corruption of the Latin catadupa for “waterfall”. From that same Latin root catadupa, we get the Spanish for waterfall…. catarata.
The c-t root that begins both catarata and “cats and dogs” does make this etymology plausible. But is it real? I just don’t know.
It does, however, make it easy to remember: a waterfall has the intensity of rain showers even stronger than with “cats and dogs”!