This character was pulled from the film, which was an interesting statement about the times. It also represents a shift from similar characters in previous Disney films.
There are the initials "WK" on the piece and the seller thought the image might have been drawn by Ward Kimball or Walt Kelly (of "Pogo" fame), but there is no specific indication of that fact. I don't have any additional information that would provide any clues. If you have any thoughts, please leave a comment....
Black Centaurette Pencil (click to enlarge)
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From Fantasia (1940), the Pastoral Symphony sequence. A pencil sketch of a black Centaurette (#45) with flower in her mouth. Notes: "#45"; the image has "WK" written on it. [Image: 11-1/4"W x 9"H] SeqID 0040
Seller thinks this stands for Ward Kimball or Walt Kelly. He thinks the nature of the image would lead him to think it's Walt Kelly -- who did Pogo.