shrieker
By itself, the shrieker is completely and utterly harmless. You can walk right up to one, and it won't even attack you! They don't move, and while they might shake a bit, they're not intelligent in the least.
However, shriekers aren't named ironically. When exposed to light, or when attacked or otherwise jostled by a creature, the shrieker emits an ear-piercingly loud trill, which attracts the attention of nearby creatures - making them appear suddenly by the shrieker, without warning.
One way to think about a shrieker is that it is basically a living mob spawner... that can spawn any hostile mob (native to the overworld), and can summon a fairly large group at a time.
Shriekers come from Dungeons & Dragons. In older versions of the game the shriek was more a defensive reaction that might alert other creatures. This seems to have been rethought in new versions of D&D.