Psalm 32:9 Be ye not as the horse, or as the mule, which have no understanding: whose mouth must be held in with bit and bridle, lest they come near unto thee.
Kings rode on mules. - 1 Kings 1:33 The king (David) also said unto them, Take with you the servants of your lord, and cause Solomon my son to ride upon mine own mule, and bring him down to Gihon:
Mules were given to kings as gifts. - 1 Kings 10:25 And they brought every man his present, vessels of silver, and vessels of gold, and garments, and armour, and spices, horses, and mules, a rate year by year.
Mules carried burdens and messages. - 2 Kings 5:17 And Naaman said, Shall there not then, I pray thee, be given to thy servant two mules' burden of earth? for thy servant will henceforth offer neither burnt offering nor sacrifice unto other gods, but unto the LORD. and Esther 8:10 And he wrote in the king Ahasuerus' name, and sealed it with the king's ring, and sent letters by posts on horseback, and riders on mules, camels, and young dromedaries:
It is in Psalm 32:9, however, that we see some not so wonderful qualities of the mule. It tells us that the mule has no understanding, not natural or innate wisdom. Stubbornness also is referred to in the reference to the necessity of the use of a bit and bridle for the mule. Without that bit and bridle the mule would not naturally come when called or be directed by the master.
There are a good many comparisons here for us as believers. We are not here for ourselves, but for the service of our master the King. The Bible tells us that we are gifts to our King. (Rom 12:1-2) At times we must carry burdens, and always we carry a message from our King to others. The problem is that we, like the mules of old, are stubborn. We only have wisdom when we look to our Master for direction, and if we had our own way we would wander off not heeding the Master's loving call.
As you can see, there are good and troubling aspects to God's comparison of us with the mule. May we exemplify the best in this comparison.