These are some of the silhouettes I did for Borderlands 2 DLC concept project. The armored robot I was designing was supposed to be made up of CL4P-TPs.
So the story we came up with was that while digging in the iridium mines they came across this sphere that activated and then magnetically pulled in any surrounding CL4P-TPs and/or loose pieces of metal to form this huge destructive robot that walks around on its hands. After killing Diamond Dan and some other mini bosses your player would come across this guy while trying to escape through the tunnels.
These are some sketches I came up with for some extra monsters/creatures you could fight.
This Sand Tuber is basically an underground venus flytrap. It waits under the sand with a weapon or loot as bait for the player to come get. Then when the Tuber feels approaching footsteps it snaps shut in an explosion of sand. Its inner jaw extends up out of the sand and chews up the unsuspecting prey. The "mouth" is made up of four bone-like edges with a thin membrane between them that spray neurotoxins when it shuts. Although it is usually rare, its not impossible to escape once you've been trapped.
The B4N-D1T0 is the six armed bounty hunter robot with 6 six-shooters. His extremely agile arms slide up and down his rib rails making for a faster-than-lightning quickdraw. With one good eye and an empty chassis in place of a heart, the B4N-D1T0 always gets his target.