Making players afraid of the dark is a nice idea. It's a concept that works very well in The Forest and also 7 Days To Die.
Personally, as I've said a billion times already, I think they should have gone with cannibals (taking inspiration from Cormack McCarthy's book, 'The Road'), but The Forest got there first. So tough luck on that front.
Whatever the choice, I think it needs to be something with an element of 'horror' - i.e. something players should be 'afraid' of. I used to spend a lot of time wandering the outer reaches of the map, just for LULZ, and I never felt any sense of danger when doing so... the environment itself just isn't threatening.
So whatever the NPC monsters do end up being, my only request is that they're something I'd be inclined to run away from. And ideally screaming whilst I do so ;-)
And they don't need to drop loot. Other players, airdrops, crates, etc, fulfill that role just fine. The NPC monsters just need to function as being a credible threat. Something to wipe the smiles from the faces of players when they reach the top of the food chain, Kelvar armour and MP4s or not.
I totally agree with is that you say! In the end, regardless of the creature, as the atmosphere is well made, with an atmosphere of fear, which forces you to flee rather than fight!
To this I say BRAVO SKOL!
(After this is true, I personally like a lot of creature and discover fauna and flora varied. Where I tell myself. "THIN, WHAT IS THIS CREATURE HOW I DO FOR THE KILL HIM OR ESCAPE ? "