Given the amount of speculation and limited quality of information surrounding some of the game's more interesting (and unfinished) enemies, I thought I would provide some definitive data which represents the current state of those which cannot be killed, whether practically or functionally.

Not every enemy is in a complete state, you can draw your own conclusions as to why, and what their intended purpose is.

• Some have absurd health values.
  
• Some literally cannot die, when you shoot them and their health reaches zero, you will get the metal-screeching-kill-confirmation sound (every time you hit), but they won't functionally die. They will keep acting as normal.
  
• Some can functionally die, but do not have reasonable death states. They stop functioning, but, for example, enter an idle standing animation, breathing and moving hands around, but not being a valid enemy.

Some of these can spawn as Hunter Killer spawns, they don't seem to have different health when they do, but they do have the behavior of hunters that causes them to be aggroed on players without regard for distance. This makes the Medium variants extremely dangerous.

• Euruskan Grabber (dog) 1B; doesn't die
  
• Euruskan Toothy Mech 100M; doesn't die, top gun can be shot off
  
• Euruskan Orgamech 1B; does die, idle animations
  
• Eurasian MotherCourage 1B; does die, idle animations
  
• Europan Medium Mech 168,750; does die, both shoulders can be shot off (rocket pods and machine gun), rear turret can be shot off

Version: 0.1.38217.0