Creep

A creep is any monster in the game or any non-quest-giving and non-shopkeeper NPC combatant.

Typically used on Battle.net on Warcraft III. The term mob is more often used in World of Warcraft.

Comes from Warcraft III where the neutral (but aggressive) packs of monsters would guard certain areas from your armies. Originally, in Starcraft, creep was a layer of slime where zergs could build similar to blight.