A saddle and a seated human may at first seem too distant to compare. The poet perceives a shared configuration: bent lines, hanging elements, and the relation of parts to a central support.
Thus the rarity belongs to the underlying simile. The hearer must mentally construct both arrangements before the common configuration becomes clear. By contrast, courage connecting a lion and brave man is already familiar.
The criterion is the originality and precision of the comparison—not the age or unfamiliarity of the vocabulary about horse equipment.