Ann Stafford was the pseudonym used by Ann Pedlar. She wrote two pony stories in the 1930s, and many other titles, some of them historical fiction. It might be that some of her other early works are pony-orientated. As they are almost impossible to find, I haven’t been able to find out yet.
Five Proud Riders is a good read: the villains are those traditional pony book baddies, the gipsies. It is probably the earliest example of the trekking story, which became a popular vehicle for pony book authors over the decades: it did after all make a change from the usual girl gets pony and wins the gymkhana plot line (Ann Stafford managed to get a show in, but does it at the beginning of the book.)
Finding the books
Five Proud Riders is very easy to find in its Puffin incarnation; easy to find in its American printing, and almost impossible as the UK first edition. Pony for Sale is also easy to find as an American edition; the Reindeer UK edition isn’t impossible, but the UK first is very hard to find indeed.