-Rare Rockworm &
-Rare Sand Dweller &
-Champion Fallen Cur &
-Champion Plague Swarm &
-Champion Savage Rockworm Are all Monsters that can not spawn and are needed for Achievements.
-Subjugator Champions can spawn, but are not checking for confirmation in the Achievement-screen.
unless I am mistaken I thought I read in the patch notes these monsters should be spawnable & achievements available, however many players are stating this is not true
I have not personally farmed enough to confirm this is the case myself, other than to say all the time i've spent I have not yet seen these monsters here, nor on any of the live streams I follow.