I went through a phase of reading pony books when I was a young teenager, and this was the one that I went through obsessively, tracing the illustrations into a notepad. I have no idea who the illustrator was for the edition I read - but I was there, with the herd, totally immersed in their world. I always preferred the stories about the horses themselves to the Pullein-Thompson/Ruby Ferguson 'girls going riding in the English countryside' books.