Upon his mule - So in 1 Kings 1:33, 1 Kings 1:38 the mule is the royal animal on which David himself rides. In 2 Samuel 18:9 Absalom rides upon a mule.
Related Commentaries of 2 Samuel 13:29
2 Samuel 13:29
29 And the servants of Absalom did unto Amnon as Absalom had commanded. Then all the king's sons arose, and every man gat him up upon his mule, and fled.