A Knocker is a type of goblin found in Welsh and English folklore, closely related to the gnomes known as Coblynau but more malicious in nature.
They are depicted as ugly little humanoids often dressed in traditional mining gear and they primarily existed to cause chaos and misery to miners via distracting them with knocking sounds, causing cave-ins or other disasters.
They could vary from simply mischief-prone pranks to outright murder.