The Disappointment, 1767, a play produced by Andrew Barton (pseud.), featured the comic character "Raccoon," described as an "old debauchee," speaking a "proto-American Negro dialect."
He was vain and very superstitious.
In 1775, in The Candidates, Robert Munford, the comic Negro character, Ralpho, appeared. He was foppish, spoke a peculiar dialect.

In character, Ralpho is a step toward "Zip Coon."
"Zip Coon" seems to be a revision of the name Raccoon.

Alan W. C. Green, 1970, "Jim Crow,": The Northern Origins of Negro Minstrelsy," The Massachusetts Review, vol. 11, No. 2, pp. 385-397.

Only the first page is on line, provided by JSTOR.