"The Horse and His Boy" is one of the Chronicles of Narnia that I never actually read as a kid. It is the third book chronologically, and the fifth book published in the series. You're probably wondering why they haven't made a movie out of this one yet; I see three possible explanations. One, the title "The Horse and His Boy" doesn't exactly sound too thrilling or like a box office hit. Two, they're following the main Peter/Lucy/Edmund/Susan storyline first (Well, also Mr. Caspian, but he's an entirely different bucket of fish.). Three, no one in the entire book shouts "FOR NARNIAAA!" thus making it cinematically inferior compared to the others.
The plot follows Shasta who is (spoiler) an exiled prince from the northern next-door neighbor to Narnia kingdom of Archenland. He was raised in the southern kingdom of Calormen (which seems to be a stereotyped version of the Ottoman empire) by a personalityless fisherman named Arsheesh. When the one of the grand Pooh-bahs, I mean, Tarkhans, comes and tries to offer Arsheesh a little baksheesh to buy Shasta as a slave, Shasta overhears this and wanders away, encountering the Tarkhan's horse, Bree. The horse reveals he can speak and that he's sick of servitude. They plan an escape to Narnia where they can both be free. In the process, they run into a female Talking Horse with a young noblewoman, who is escaping an arranged marriage with the ancient vizier.
In the process of their escape from Calormen, Shasta and the girl Aravis learn of the headstrong prince Rabadash's plot to attack Archenland and Narnia so he can capture Queen Susan and force her into marriage.
Whenever the book mentioned Rabadash, which was often, I couldn't help picturing this:
Rapidash, Rabidash: not the same thing. Rabidash is actually supposed to look a lot like this:
So yes. The good guys win (with liberal help from Aslan) and Shasta is restored to his true heritage as crown prince Cor of Archenland. Hooray!