Silverfish

The Silverfish is a new mob added in Beta 1.8. Silverfish hide in stone bricks found only in strongholds, and pop out to attack when the blocks are mined. In regular gameplay, Silverfish blocks occur rarely. If left alone, silverfish can re-enter some blocks, including cobblestone. When attacked, Silverfish may awaken nearby Silverfish blocks. This can lead to a terrifying swarm if there are enough in the vicinity, and in which an uncontrolled swarm can quickly destroy dozens of stone bricks within a Stronghold. They scurry away after the player moves a short distance. They have the same sounds as spiders, which is a placeholder.

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, although they can also appear elsewhere.

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.
 * There are also small ray-finned fish called Silverfish. These were proven unrelated after a tweet from Jeb.
 * The block they spawn from when destroyed is ID 97, when placed it will have the texture of regular stone.
 * Block 97 is mined faster with hands than with tools.
 * As of 1.8 They spawn additional Silverfish from nearby Silverfish blocks (Block 97) and blocks containing escaped Silverfish when attacked. Blocks are destroyed in the process of spawning.
 * Silverfish make the same noises as Spiders in 1.8. This is a placeholder and new sounds were made, but not yet implemented.
 * 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.
 * Silverfish will eat through wood and possibly other blocks.
 * Silverfish can be beaten naturally by using fire without it calling in more of its allies, so long as the player does not hit it directly.
 * Silverfish can ride in minecarts like all mobs but only seem to get into them when jumping, they push them around when just crawling into them.
 * Silverfish can be killed by explosions without more spawning.
 * The stone-like block that produces silverfish when broken is often used in some servers as a form of griefing, in which the griefer places the stone-like blocks in players' houses, and the player gets attacked by silverfish as soon as they break it.
 * Silverfish do cause lag when in massive groups, enough to crash a server. (tried and tested)