The story
From plat, a term of unknown origin, plus eye. Attested from the 19th century. Originally African-American, especially South Carolina.
Etymology from Wiktionary, CC BY-SA 4.0.
plat-eye
Today it means: A mythical monster or ghost in the folklore of the West Indies and southern United States; it is a being with large, glowing eyes, capable of shapeshifting and sometimes depicted as a phantom bound to a particular place, such as a cave or grove, as a guardian (for buried treasure, etc). Full definition →