So, a quick insight to some of the Monstrous enemies you'll be encountering in the Wasteland that we've got in-game, and working.
The Gelth will inhabit the bodies of the nearby deceased, providing there's a corpse that's been rotting for less than 12 hours. Upon finding a successful host, the Gelth will inhabit the corpse and rise as a 'Gelth Cadaver', causing zombified chaos and havok in the immediate vicinty. Gelth Cadavers are incredibly prone to fire based-damage.
Auton Duplicates are slightly more rare, but will spawn in NPC populated areas. Duplicates, by definition, are sleeper agents working for the Nestene Conciousness. As such, they could be anyone around you, and thus could spring the trap and go on the attack at any second.
As an example, I was minding my own business wandering through the Ultra-Luxe, when a very charming looking young woman, who had been minding her own business as I wandered through beforehand, spun on her heels, whipped open her wrist-gun and begun firing like a possessed Plastic dummy. A swift deployment of Antiplastic and the Duplicate was felled.
So there you have it, just an example of two of the currently implemented creatures within game so far. We've got mechanisms and issues to handle, but we'll be crossing these off as we go along.