For example you might stumble across an onix emerging out of the side of a cave, and then probably burrowing into another
If you added particle effects To where it was entering or emerging from a wall and made onix vibrate it could look quite good
I'm posting this cause I don't otherwise see how an accurately sized onix could fit into a cave system
Properly
Edit: every one likes the idea of destroying blocks while tunelling but not the aftermath so how about when onix destroys a block (best if it can only destroy smooth stone) the blocks are replaced by a block that is identical to an air block however they slowly decay back into smooth stone (much like how leaf blocks slowly decay into air blocks but this would be the other way round) and also when onix moves through actual air blocks nothing changes.