Yesterday I wrote up a unique monster for my campaign, a Gargoyle named Slate. The name isn't original, but precious little is these days.
It didn't occur to me until today that I had basically turned the monster into an NPC.
Really, what is an NPC? It's a monster interesting enough for the players to interact with prior to or instead of killing it. Because lets be honest, most groups in fantasy RPGs are thinking about the body count ;)
If the highwaymen don't have names, they're monsters. If they have names and motivations they are NPCs.
A gargoyle is a monster, unless it's named Slate and can converse in the common tongue, at which point it becomes an NPC.
Now, if the NPC becomes a DMPC we have a whole 'nother problem ;)