Sue Garnett wrote just the one pony book that I have been able to find. Cassie, a child of the Vicarage, has to move when her father moves to a different parish. Although published by a Christian publisher, this book is a move away from the evangelistic pony books of the 1960s, avoiding their overt religious tone.
Finding the book
A little bit more pricey than the average pony book, but not impossible to find.