Cave Krane

Cave kranes are large subterranean creatures with large pointed shells and sharp tentacles which they use to burrow around.

Details:

A cave krane would be encountered only below y level 50ish, they tend to stick around large ore deposits or water veins, so an encounter with a krane usually means a large deposit is nearby. Kranes are mostly stationary but their tentacles would chase after you destroying blocks in their way. A kranes blue and black shell surrounds most of its back and is immune to arrows, but they are weak to fire or lava, and pouring some near its position is a good way to kill it. They are hostile and make a large grumbling noise when you get within 20 blocks distance and their tentacles will begin to chase you through the rock at a distance of 12 blocks. fortunately they are uncommon and there are no more than 1 per 3-4 chunks, spawning more frequently in wet caves under swamps and oceans. The Cave Crane often drops flint (from all the stone it grinds up, as well as a krane claw, a single use item that digs up to 12 blocks in a row in any direction you point. 

Photo is a work in progress as not many live to describe the encounter.