AspectHorse verseCaravan verse
Underlying likenessArrangement of tack and iḥtibāʾFast travel and flowing water
FamiliarityUnusual and subtleInitially familiar
Cause of rarityOriginality of simileAssigning sālat to the valley floors
Both may count as distinctive metaphors, but not for the same reason. Identifying the mechanism is more valuable than memorizing the label by itself.