Black Beauty wasn’t the earliest pony book. The earliest one I’ve found is The Memoirs of Dick, the Little Poney, which was published in 1800.
Patricia Leitch also wrote as Jane Eliot. Under that name she published four stories: Pony Club Camp, First Pony, Afraid to Ride and Jacky Jumps to the Top.
The Pullein-Thompson sisters were not the first pony book writers in the family: their mother, Joanna Cannan, wrote classics like A Pony for Jean. It was published in 1936, and was one of the first books to concentrate on the rider rather than the pony.
Moorland Mousie, the equine hero of Golden Gorse’s books, really existed.
Caroline Akrill, author of the Showing series, was once banned from the Ponies of Britain show by its founder, Glenda Spooner (they later made up).
Mary Treadgold wrote her wartime classic, We Couldn’t Take Dinah, while sheltering from bombs in the London underground.
Josephine Pullein-Thompson wrote Six Ponies on the roof of the telephone exchange at Reading, where she was doing war work.
Christine Pullein-Thompson worked with horses in America, which gave her the background material for her Phantom Horse series.
Babette Cole, author of the Unicorn Hall series, was a famed side-saddle rider, and breeder of horses and ponies.
The inventor of Parkinson’s Law, C Northcote-Parkinson, wrote a pony book called Ponies Plot. It was based on his children’s wicked ponies.
Mario Puzo wrote The Godfather. And he also wrote a children’s horse story: The Runaway Summer of Davie Shaw. Peter Harrington, the London-based antiquarian book dealer had for sale a first edition presentation copy of The Runaway Summer, in which Puzo wrote, “For Carol. If you don’t like it, I’ll kill you.” Perhaps an intimation of things to come.