Guest,Jim:
"Hound dog" is not redundant. Anyone interested in dogs (for breeding, showing etc.) will know that the Kennel Club lists 6 'Groups' of dog breeds:
Utility, Working, Toy, Terrier, Gundog & Hound

So "Hound dog" is just a little more specific than just "dog", however, the wording may be mis-understood. If you read the Wiki description of a hound you may consider the song trying to tell us:
"Dog, you are nothing but a hound"
Other definitions make it clear that 'Hounds' as a group will hunt game, but act only as hunters (or 'pointers') Making a lot of noise (baying) but NOT acting as retrievers.
This would also make sense of the the lines "Crying all the time" & "you ain't never caught a rabbit"


CHEERS
Nigel