MineSkorp: 'I bet they pick up materials and build weird stuff. When you come outside in the morning you'll be like "WTF? Where did this stuff come from?" Or maybe not. I dunno. They might make traps that kill you, then take all your stuff.'
you know I would agree with MineSkorp because they appear to be intelligent to some extent, who knows, maybe their some kind of evil NPCs not Evil Mobs
also on another note, in the third image it's actually 2 images put together.
WELL. I think I have a small theory (if even THAT) about who (or what) they are. Think about it. White eyes. One of them was holding sand. [SAND PYRAMIDS] This is a pretty big strech, but it's creative. THE NEW MOBS ARE HEROBRINE'S DARK ESSENCE. Notch deleted Herobrine. But Notch could say that 'He' somehow emerged again as shadow versions or something! [WINNING IDEA]