Albert Barnes Commentary 2 Samuel 13:29

"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." — 2 Samuel 13:29 (ASV)

Upon his mule - So in 1 Kings 1:33 and 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.