وَأَخَافُ أَنْ يَأْكُلَهُ الذِّئْبُ
I fear that a wolf may devour him.
Here «الذِّئْبُ» need not designate one individually identified wolf known in advance. The fear concerns an animal of that kind that might attack. Thus a noun grammatically marked as definite can function through mental familiarity without specific external identification.
This is a standard illustration of mental reference with al- (لام العهد الذهني). Reading it as exhaustive inclusion would wrongly suggest that all wolves collectively perform the action, something the context does not require.