Bedrockgames
I post in the voice of Christopher Walken
It seems to me that if the GM can introduce unique entities - perhaps in part by establishing additional fictional elements to make them fit in (like Gollum is far too weird and long-lived to be a Hobbit, but this is the result of him having had the One Ring all that time)
Just to make one final point on this. Ultimately this is about NPC and PC creation and that is about preference. My preference here, at least in terms of a typical fantasy RPG (it certainly would change for certain systems of play and with certain groups I game with), is that the players have a set character creation method which wouldn't generally allow for this sort of thing, but that NPCs and Monsters wouldn't be expected to follow the same rules of creation as PCs all the time (certainly many NPCs would follow the same rules, but the GM needs more liberties in my opinion to make the campaign interesting and going the direction that 3E did, where there is a lot more parity between how PCs function and how monsters and NPCs function, was a pretty big headache in my opinion). But again that is just a matter of preference.