When tribes were encamped in an area that overlapped the lands of their enemies, extra precautions were taken to insure their safety.  Alert young warriors were strategically placed to watch through the night for danger and possible war parties striking at night, the best time to steal and run off horses.  It was a lonely job.  I know because I pulled night sentry duty many times in the Army.  No horses were stolen during my time on the perimeter.