Now these things would be Neutral, and would spend most of their days sleeping. They look like water bears, if you don't know what that is, look it up. Now if you attack them they will wake up and shoot rocks at you(Think Octorock from zelda). They Drop Tawnyskin Which is a Greyish leather looking thing. It's tremendous fat reserves make it so just one skin can melt 40 Blocks! However, they are quite rare. (Think wolfs.) Overall they would add just a touch of horror to the nether. Tell me what you think!
Edit:Pic! http://imm.io/a4jo
It seems pretty cool, i dont know about the shooting rocks part, with that and dodging ghast fireballs seems a bit much, maby they could do a ram attack that would knock you back a couple of blocks
Edit:Pic! http://imm.io/a4jo
It isn't the best of pics... but I get what you are saying.
You should try get firehazurd to make a mock up of your mob, he is the leading mob designer on the suggestion forums.