It's funny you should say that, for as I was rereading this, I was noticing the many similarities between this scene and the way it was presented in S&S2. Even the part about Marianne declaring it would not rain and chasing the blue sky, etc.--it's all there, except for the white horse.

Here's the relevant bit from Emma Thompson's screenplay:

As [Margaret] goes into the mist we hear the thunder of hooves. CU Margaret's terrified expression. They seem to be coming from all around. She wheels and turns and then - Crash! Through the mist breaks a huge white horse. Astride sits an Adonis in hunting gear. MARGARET squeals. The horse rears. Its rider controls it and slides off. He rushes to MARIANNE's side.