Heritage Auctions are currently offering an original model sheet of Cinderella's Fairy Godmother. Their description doesn't include the name of the artist, but it is clear to me that this is the work of Marc Davis.
This character started out originally as a realistic figure during the storyboard phase of production. Compatible to the look of the Stepmother. Milt Kahl lobbied for a screwy, eccentric, funny concept, but Walt didn't see it that way. These Marc Davis sketches show a compromise. There is realism with potential for some eccentricity. So Milt took it from here. I do like Marc's abandoned idea of the Fairy Godmother hovering above ground (sketch #3).
Here are a couple of links to earlier posts about Milt Kahl's Fairy Godmother: