ما أهر ذا ناب إلا شر
Nothing but an evil cause made a fanged creature growl.
The negative construction with إلا شر states a restriction involving the cause. The discussion cautions against treating the negation as a denial of every possible action by the animal: the utterance concerns a particular cause and its resulting response. The indefiniteness of «شر» can contribute to the impression of gravity.
Two devices are at work: restriction through exception and a rhetorical effect related to ordering and indefiniteness. Their contributions should not be collapsed into one.