Island Head Beast

The Island Head Beast is giant floating humanoid like creature with a mouth of crooked teeth, attacking anything that enters it lake.

History
The Island Head Beast is hideous monster that dwells in the lake of Gravity Falls, as the twins; Dipper and Mabel Pines find one of its teeth on shore they decide to investigate on it and head into lake around night on a boat. While they seek out the creature, it suddenly raises from the water and approaches towards them. The twins barely mange to escape as it took a bite out of their boat and lost another tooth before going back into the lake.