Guardian

The Unknown Hostile Underwater Mob is a mob that Jeb is working on than will appear in 1.8. It will be hostile and will only spawn underwater. It will not be a shark or a barracuda.

The new hostile mob will spawn in water dungeons containing prismarine and sea lantern.

It will be a rather "nasty" mob. Its health, attack strength, drops, model and name are all unknown.

The mob will have a ranged attack and will be hostile to squid as well as the player. A full set of diamond armor, potion of water breathing and respiration enchantment is recommended on hard mode.

Trivia

 * An image sprite of the mob was uploaded to the stylesheet of the official Minecraft subreddit under the name, "Plankton". The moderators have since stated that it was due to the fact that they had no idea what it was either.