Generic NPC. See generic_actor.
Directly spawning this ingame will either crash the engine (Team Fortress 2) or report an error in console (Model Name is Null)
Appears to be unsupported and replaced by generic_actor. Use this at your own risk.
Note: It is possible this NPC is used as a base to code new ones. Another theory is that this entity was passed down from Half-Life, as most NPC entities in Half-Life began with 'monster_'. If this is the case, the way this is unsupported can be compared to that of func_wall, or func_illusionary.