This mythical creature is of Welsh/Cornish origins. Said to be 2 feet tall, unshaven, and wearing miner's garb, they are associated with the worst situation - a cave-in. Miners created the story when they noticed knocking that occurred before cave-ins. Some believed it was a warning to get the miners out, others believed their knocking created the cave-in. Miners who lost tools often blamed it on the Knockers. For such a very risky occupation, superstition and explanations for loses and danger took the form of these mischievous beings.
Gargoyles are seen carved into many older structures. They are believed to have originally been a device to divert rainwater from running into the building. The sculptures became quite the rage on buildings and took on a popular belief they protected a building. They were said to scare evil away from churches. Even though they are no longer the rage on houses, they are extremely popular in gothic decorating and Halloween sets.
This is my personal favorite gargoyle representation in entertainment, a 1972 movie entitled "Gargoyles."