مَنْ فِرْعَوْنُ؟
Who was Pharaoh? (emphasizing his terrifying stature)
In the reported interrogative reading man is an interrogative noun and Firʿawnu is nominative. A syntactic account treats Firʿawnu as subject and man as predicate, with the reverse analysis also discussed. The force is to magnify the notoriety of Pharaoh's oppression, not to request his name.
Man hādhā can belittle a known person; man Firʿawnu, in this context, can intensify the sense of dread. The distinction depends on context and intended impact, not merely on the interrogative word.
Rhetorical useInterrogative formCommunicative effect
BelittlementMan hādhā?Disdain for a person
Magnification of dreadMan Firʿawn?Sense of danger and severity
Genuine inquiryMan hādhā al-gharīb?Learn an unknown identity