Margaret Courtney, in Cornish Feasts and Folk-Lore (p.129), says:

Bucca is the name of a spirit that in Cornwall it was once thought necessary to propitiate. Fishermen left a fish on the sands for bucca,
and in the harvest a piece of bread at lunch-time was thrown over the left shoulder, and a few drops of beer spilt on the ground for him, to ensure good luck.

He seems to have declined from a godling to a hobgoblin, for she further says:

Bucca, or bucca-boo, was until very lately (and I expect in some places it still is) the terror of children, who were often, when crying, told that 'if they did not stop he would come and carry them off'.

She also says that there were two buccas: Bucca Dhu and Bucca Gwidder. One version of a 'Mock Ghost/Real Ghost' story is given by Bottrell in Traditions and Hearthside Stories (Vol.I, p.142), as 'The White Bucca and the Black'.

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