auf den Hund gekommen

Definition

Is quite analogous to English (his life) "went to the dogs". Originally, the dog and Hund in these expressions had nothing to do with a 4-legged dog. Old English docga represented the sound of Semitic shin-kuf-aiyin at a time when the shin had a dental D/T-sound and the 3 = aiyin had a velar G/K-sound, as in 3aZa = Gaza. The meaning of shin-kuf-aiyin is "to descend".

So, the dogs in "went to the dogs" and (raining) "cats and dogs" are very related.

Raining cats and dogs is first attested as raining "polecat and dog". This represented the sound of the Semitic phrase maBooL GeSHeM SH'Qi3a. Treating shin as T/D and aiyin as G, the BLGTMDQG sounded quite like polecat an' docga