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Back in November 2003, it was released One Piece Color Walk 2, where Oda drew the backs of some of the future villains that the Straw Hats were going to encounter in the Grand Line. One of them turned out to be an early design of Boa Hancock, revealed 5 years before her formal introduction; and there is another guy with horns that looks pretty similar to the appearance of the recently introduced Kaido. Could that be an early design of the Yonko? Look at the similarities closer here:

Villain conceptKaido Manga Infobox