Sylvia Scott White wrote two pony books I know of. Her Pony Pageant was part of the Crown Pony Library. The other title, Ten Week Stables, although published by Lutterworth, who produced the Crown Pony Library, was not part of it.
George Bowe, who illustrated the books, contributed to one edition of Swift, and illustrated two books by Enid Blyton.
Finding the books
Both books are easy to find, and generally cheap.