Silverfish

The Silverfish is a new mob added in Beta 1.8. Its texture was originally found in the Beta 1.7.3 minecraft.jar. In the Beta 1.8 pre-release, Silverfish popped out of stone bricks found in strongholds. If the player mined these blocks Silverfish may pop out and then attack them. A block with the same texture as stone can also be spawned in through commands that can be mined far faster than regular stone and when broken always released a Silverfish, however this is merely a placeholder block and are encountered naturally but VERY Rarely. Silverfish will also spawn near others of their kind every time the player damages one of them which can cause a swarm very easily if not controlled (Each time one spawns, it breaks the block it came out of without leaving the drop, making it so that an uncontrolled swarm can quickly destroy tons of stone brick within a Stronghold). They scurry away after the player moves a short distance. They also have the same sounds as spiders, which is most likely a placeholder.

If left alone, silverfish can re-enter some blocks, including cobblestone.

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 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.
 * As of Pre-1.8 They spawn 1-5 additional Silverfish from nearby Stone, Stone Brick, Mossy Stone Brick, and Cracked Stone Brick when attacked and destroying the block in the process of spawning.
 * 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 drown in water when forced under by a block above or something similar.
 * Despite what they are in real life, silverfish cannot break or "eat" through wooden (or any) blocks.