Delta Worms/Basalt Worms.

The basalt delta is by far my most favorite biome. It is already quite rugged and dangerous but I think that the local fauna could use a bit of improvement. The Delta Worm or Basalt Worm would be a rare ambush hunter with 12-14 health points. It would have a texture that is slightly different from the surrounding blocks, but not too noticeable. It would wait underground until a player is within 3 - 4 blocks of it and the rise up to be 4-5 blocks tall and attack. (4 blocks=12 hp +3 block detection radius and 5 blocks=14 hp + 4 block detection radius.) When killed, it leaves behind a hole that has a 10% chance of having nether scrap at the bottom. 10% is not too high, due to the fact that they will be quite rare. The attack could be fireballs or ramming it's head into the player, but I am sure that the team at Mojang can come up with something interesting.