Shelob is a Spider special NPC, there is no known difference in psychical appearance, however Shelob has about twice the health than a regular spider. Shelob can be found wandering the swamp levels, or inside of a small room with spike trap floors inside of the mines. (Pulling 1 lever will trigger all the traps, pulling the second lever will open the gate)
Shelob will drop Dyrnwyn, the magic sword.
TriviaThe name Shelob is a reference to the great spider from the Lord of the Rings.