Silverfish

Silverfish are a planned mob, as of now only a skin file can be found in the minecraft.jar file, but a possible 3D recreation has been made by a forum user called 'sunflower' Silverfish could be used as an eating mob. This would mean the silverfish could destroy Wood, Bookshelf, and Wooden planks.

History
The image, silverfish.png, was added to the /mobs/ folder in minecraft.jar in the Beta 1.7 update.

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 named Silverfish.
 * Silverfish could possibly be the "scary mob" that Jens was working on.
 * This may be the fish that Notch wanted to add for fishing in the Halloween Update but didn't have time to. This may mean that it is an actual fish and that the "silver" describes the colour of the fish, although this won't explain the model by forum user 'sunflower'.