Silverfish

The Silverfish is a new mob that is planned for Beta 1.8. Its texture can currently be found in the Beta 1.7.3 minecraft.jar. Silverfish will pop out of Stone Bricks found in Strongholds. If the player mines these bricks, Silverfish may pop out and then attack them. Silverfish will also spawn near others of their kind, which can cause a swarm if not controlled. They scurry away after the player moves a short distance.

History
The image, silverfish.png, was added to the mobs folder in the Beta 1.7 update. The silverfish mob was later announced for 1.8.

Jeb confirmed that the silverfish won't be a water mob, ruling out the possibility of the Silverfish being an actual fish.

In the Beta 1.8 update leak, Silverfish can be found in Stone Bricks in Strongholds.

Trivia

 * In real life, silverfish are small, wingless insects that are often found in dark areas in buildings. They are infamous for damaging wood and paper-based products.