Template:Mob
The image, silverfish.png, was added to the /mobs/ folder in minecraft.jar in the Beta 1.7 update. It is unknown whether the silverfish is a mob or not. It may be added in the 1.8 update, as Jeb said that he would be adding a new "scary mob" to the game. In real life, Silverfish are common small wingless insects that are often found in dark areas in buildings, and are infamous for damaging wood and paper-based products.
References
Forum Post: http://www.minecraftforum.net/topic/414409-new-mob-skin-silverfish/