Glow Wyrm

The Glow Wyrm would be a semi-neutral mob living in underground lakes. In the Minecraft Live trailer, I felt that the underwater lakes were a little empty. I think that a leviathan would fill that void.

The Glow Wyrm would be a big dark blue sea snake with many glowing fins and spots on its body, similar to the real-life viperfish.

The Glow Wyrm would be very territorial and would lash out at any mob that gets too close, though it would hunt Glowsquids and fish outside of its territory. If two meet each other, they'd fight. It would only attack the player if they entered the water and immediately stops when the player leaves unless the player provokes it. They get distracted easily by boats.

When fighting a Glow Wyrm, they have a bite attack and an attack where its fins on the sides of its head open outwards and inflict blindness on any player looking at it head-on. The bite attack would do 2 hearts of damage, and it would do more armor damage. If a player were to attack it on land, it would climb out of the water try to attack, needing to return to the water after 10 seconds. When killed it drops its glowing fins, which can be used to brew potions of blindness or craft sea lanterns, and it's teeth, which can be used to make arrows or be used to make more long-lasting potions of water breathing.

You can earn a Glow Wyrm's trust if you find one stuck outside of water, helping it move into water will prevent it from attacking you when you enter its territory.