Cave flatfish

These mobs would be a harmless hostile mob like the tiny slime this mob would be difficult to spot the only sign it would show is a darker block of stone traveling through the bottom of the cave ground toward you it would get under you pushing you up and popping out of the ground it would resemble a grey Shulker with a different face in it. It would not be able to travel through other blocks besides stone. I think this would be great for puzzle and adventure maps and even elevators!