Sentient Stone

Picture this: You're walking down a cave and see a stone block that is a bit out of place. You pay it no mind and walk past it when WHAM—you get flung across the cave. You turn to see what in the world just happened, and you see the conspicuous stone from earlier has spurt little arms and legs and is now lumbering quickly towards you. These little fellas can only be damaged by a pickaxe, so you better be prepared. If you manage to get far away enough for it to lose interest, it will retract its limbs and go back to being a solid block. It is also immune to knockback and has a small chance to drop ore when killed.

Why: There should be more underground mobs in the game to make cave exploration more interesting. Additionally, block-based mobs suits the identity of Minecraft, shulkers are a testament to that.

Side note: Alternative names could be Stone Golem or Pebblers.