As far as I know, Roy Brown wrote just the one book which could be defined as a pony book: A Nag Called Wednesday. He did write extensively for children.
A Nag Called Wednesday is about a lost horse who happens to belong to the local knacker, who wants him back. Liz and Larry are determined to save the horse and hide him until he can be found a good home. The book is out of print, but easy to find.