Silverfish

The image, silverfish.png, was added to the /mobs/ folder in minecraft.jar in the Beta 1.7 update. It may be added in the 1.8 update, and it may be the "scary monster" that Jens was working on. Nothing else is known at this time.

Real Animal
Silverfish are small, wingless insects that are often found in dark areas in buildings. They are infamous for damaging wood and paper-based products.