The Bucca is a sea spirit found off the coasts of Cornwall. Fishermen felt a need to propitiate it by leaving three fish out on the sands, in order to have a good catch of fish.
There are said to be two Buccas, the Bucca-du (Black Bucca) and the Bucca-widn (White Bucca).
Some think the Bucca was originally a sea deity that degenerated over the years into a fairy spirit.
W.Y. Evans-Wentz, The Fairy Faith in Celtic Countries, pp. 164, 175.