This movie looks like it was from the 60s, and probably in England due to accents. There was blonde haired boy about 11 yrs old. In the end, his horse is stuck in quick sand, and he tries to rescue the horse, and he gets stuck. I think someone pulls them out at the end with a rope or something. The movie has a very foggy setting.

Run Wild, Run Free (http://www.imdb.com/title/tt0064909/)

Thanks, that is the movie