Celestia barked at Luna, receiving a thwack on the head in return from the feline. Discord sat in the corner munching on popcorn buckets because that's just what he does. Also, he teleported all the former ponies into the air seconds earlier, so now it was quite literally raining cats and dogs.
And that's where the saying comes from. I think.
The end. ¯\_(ツ)_/¯