The Kikkuli text is one of two surviving ancient texts about horse care and training (the other being Xenephon's "On Horsemanship" from 350 BC). The Kikkuli text is a Hittite text from around 1400 BC that describes the proper care and conditioning of chariot horses. Wikipedia has a description of the book and it's significance, saying:
The Kikkuli Text addresses solely the conditioning, not education, of the horse. The Mitannians were acknowledged leaders in horse training and as a result of the horse training techniques learned from Kikkuli, Hittite charioteers forged an empire of the area which is now Turkey, Syria, Lebanon and Northern Iraq. Surprisingly, the regime used 'interval training' techniques similar to those used so successfully by Three Day Eventers, Endurance riders and others today and whose principles have only been studied by equine sports medicine researchers in the past 30 years. (link)
There does not appear to be a translation available online anywhere, although there is a partial translation on the Kentucky Horse Park's site, here. Reportedly several people have followed this method as an experiment, and had great success. Thus various folks now offer translations for sale, suggesting this ancient system of interval training might be just the thing for today's eventing, endurance and race horses.