Both of these American Girl movies skew young, but if she’s horse-mad she probably won’t care:
**Felicity **is set in colonial times (with a VERY young Shailene Woodley) and **Saige Paints the Sky **(yes, spelled that way) is set in modern times in the American southwest. Both feature riding and horses fairly prominently in the plot, although less riding than something like Hidalgo or National Velvet.
Along the same lines is a real oldie: The Story of Seabiscuit, starring a just-barely-grown-up Shirley Temple. Cute, but dated. Has actual footage of the Seabiscuit-vs-War Admiral (?) race, if I’m remembering right.