mule: the hybrid of a horse and a donkey, used as a riding and pack animal and prized as a royal mount
A mule is the hybrid offspring of a horse and a donkey, and in the Hebrew Bible it is above all a prestige animal. Kings and their sons ride mules (David's sons in 2 Samuel 13:29, Absalom caught by his hair while riding one in 2 Samuel 18:9), and foreign rulers send mules to Solomon as tribute year after year (1 Kings 10:25).
The mule sits between two more familiar animals of that world: the everyday חֲמוֹר, the workaday donkey of ordinary people, and the סוּס, the horse of war and royal display. Mules carried people and goods over rough ground, turn up in trade lists (Ezekiel 27:14) and in the tallies of returning exiles (Ezra 2:66), and stand among the other beasts in scenes of both plenty and plague.
One memorable use is a warning: Psalm 32:9 tells the reader not to be "like the horse or the mule, which have no understanding," a stubborn creature that has to be steered by bit and bridle.