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Ponies, ponies, ponies
Betty Biesterveld wrote books aimed at the younger reader, including some for the cartoon characters Tweetie Pie and Sylvester. Her one horse story was Six Days from Sunday.
Finding the book: reasonably easy to find.
Links and sources
National Library of Congress
Bibliography - horse books only
Six Days from Sunday
Rand McNally, Chicago, 1973, 74 pp, illus George Armstrong
In six days from Sunday, young Navaho Willy must leave his family and his tribe, and his ponySpotted Coat to go to school.