The boys returned the horse because they were conscience stricken. When the farmer, John Byro, said that the horse, which the boys had, was the twin of his own horse and that the people belonging to the Gharoglanian tribe were honest and trustworthy, the boys wouldn't have taken his horse. After listening to this, the boys felt conscience stricken and that the belief and trust of the people on their tribe should not be spoiled, they went and kept the horse at its true owner's stable.
Hope this helps u........ :)
