Silverfish

History
The image, silverfish.png, was added to the /mobs/ folder in minecraft.jar in the Beta 1.7 update. Silverfish could be used as a eating mob. This would mean the silverfish could destroy Wood, Bookshelf, and Wooden planks.

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.
 * There are also ray-finned fish called Silverfish.
 * Silverfish could possibly be the "scary mob" that Notch mentioned.