The horse road / Troon Harrison.

Enhanced descriptions from Syndetics:

Kallisto loves her horses. She is an excellent horsewoman, as she has been taught all her equestrian skills by her nomad mother. One day she is returning from racing her horses when she sees from a mountain pass that the Chinese army is approaching her city - clearly planning to steal their horses! She races back home to find a deserted city and her favourite horse, Swan, taken by the Chinese, along with all the other horses.Kallisto is determined to get the horses back for her city, including her beloved Swan. She bravely approaches the Chinese general and challenges his best horseman to a duel of horsemanship and skill - if she wins, she will get Swan back. Kallisto loses (she is only 13!) but the general is so impressed by her audacity and bravery that he gives her Swan anyway, on condition that she sends him a foal every year. Set in Ferghana, Central Asia, in 102 BC, this is a thrilling tale that weaves in real events. The Chinese did invade and besiege towns to acquire the famed and beautiful Akhal-Teke horses - which led eventually to horses being traded for silk and the famed Silk Route being formed. A fascinating taste of another culture and another time, couched in a richly descriptive and very pacy narrative.