Silverfish

Silverfish are a planned mob whose skin appears in minecraft.jar.

History
The image, silverfish.png, was added to the mobs folder 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.
 * It is possible that silverfish are a planned addition to 1.8. It is also theorized that they are the "scary mob" referred to by Notch being added in 1.8. Some say enderman could be the "scary mob", but this is most like not the case, since they were made by Notch, whereas Jeb made the "scary mob".