Why do the Zhaunagush call Saul's people “horse thieves”?

From the text:

"When the Zhaunagush came they brought the Horse with them. The People saw the Horse as special. They sought to learn its medicine. It became a sign of honour to ride these spirit beings, to race the wind with them. But the Zhaunagush could only see this act as thievery, as the behaviour of lesser people, so they called us horse thieves. "