If she likes books, there are some really great ones out there for kids. Anything by Marguerite Henry (King of the Wind, Misty of Chincoteague, Born to Trot, etc.) or the Horse Diaries series (history + horses, easy readers with good illustrations throughout), The Black Stallion series by Walter Farley).
Some great horse movies: Pharlap (it's sad, though, the horse dies, but it's still an amazing story), National Velvet, The Man From Snowy River, Hildalgo (but realize that despite the fact they say its a real story, it is all based on the disproved writings of the main character), The Black Stallion and The Black Stallion Returns.
Dover publishing has lots of horse stuff - books of mazes and activities, coloring books, stencils, etc. We buy the activity books & such for our little one (who loves dinos).
A set of the little Breyer horses would be great for some imaginative play.
If you have the money, I would strongly suggest getting her riding lessons before you buy her her own horse. Or, enroll her in a 4H horse program.