Lanefan
Victoria Rules
Which means the status of an individual creature can change, under your definition. For example Brugor might be an NPC but if he gets killed and the PCs end up capturing and questioning Rugit, who was previously the third goblin mook in the left flank. This interaction changes Rugit's status from monster to NPC.It might not be the official definition, but I consider an NPC to be any GM controlled unique creature. Mooks and fodder and most non-intelligent creatures typically don't fall into that category. Non-player character. To me, that infers personality. Those dozen goblins attacking? Nope. Brugor, the goblin leader? Yep.
I'm not sure that the status being able to change on the fly like that helps any with what we're trying to answer here, but it's worth noting in any case.