Silverfish

Silverfish are a planned mob whose skin only 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.
 * A user on the Minecraft Forums used the skin to construct a 3-dimensional version of the mob. The picture is still pure speculation and no official info of the mob's shape is available.