Wendy Douthwaite wrote most of her pony books in the 1990s, a period when I was taking a break from my longstanding relationship with the genre. Douthwaite's probably best known for her Polly series, about a grey Arab mare, but she also wrote a few standalone pony books. Christmas Pony is a gentle story about a bereaved girl who saves up enough to buy a foal.
You can read more about Wendy Douthwaite here.
Read more about the pony book in Heroines on Horseback, my survey of the world of the pony book from the 1930s onwards. You can buy a signed copy from me (I'll post them worldwide). The book's also available from the usual sources, or your local bookshop.