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. In the Beta 1.8 pre-release, Silverfish will pop out of an unnamed block that holds the same texture as Stone found in caves. If the player mines these blocks, Silverfish may pop out and then attack them like so. 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. They also have the same sounds as spiders.

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.
 * The block they spawn from when destroyed is ID 97, when placed it will have the texture of regular stone but with a faster mining rate.
 * Silverfish make the same noises as Spiders in the pre-release version of 1.8. This may be a placeholder.
 * Silverfish will suffocate and can die if they crawl through a Portal block.
 * Silverfish do not seem to drown in water, despite swimming underneath the surface when submerged.