This SNPC is a giant spider. It kill's you in one hit but it doesn't have an attacking animation. The sounds are models are used from Dark Messiahs Spider Queen boss. It is said in the Garrysmod org description, you might need Nuke Pack 4 or any explosive weapon to kill it because it has ALOT of health! I say that this is a boss. To get the model, find the folder in the browse section.
LINK FOR THE SPIDER SNPC:http://www.garrysmod.org/downloads/?a=view&id=125561