Fa-alḥaqnā comes from the Form IV verb *alḥaqa*, “to cause to reach or join.” The suffix -nā marks a first-person plural subject. The context is the riders reaching the quarry through the horse’s performance.
Bi-l-hādiyāt identifies the animals being reached. The following clause, wa-dūnahu jawāḥiruhā, supplies a second spatial fact about those left behind. The syntax first presents the achievement and then fixes the relative positions.