2001-03-13 read to bring back childhood memories

I first read this book when I was about 7 or 8. I think it was originally written back in the 1950's, although I have heard that the ponies are still rounded up yearly and swim across from one island to the other. This is a classic tale of 'two brave kids' who tame a wild pony and care for her and her foal (Misty). These days I wouldn't dream of letting my daughter ride bareback without a jockey skull and only a halter for steering but I can remember when we were less focused on safety and more into just enjoying the freedom our ponies gave us. This book reminds me of being a kid, riding like a looney and having a great time. The story as I recall leads up to a horse race which the boy-hero wins on the Phantom. Of course, in true Hollywood style she then escapes and swims back the island leaving behind her filly. Sob, sob. I think there was another book after this one about Misty and her foal? And possibly a film in the 60's? Does anyone know??