Albert Barnes Commentary Esther 8:10

"And he wrote the name of king Ahasuerus, and sealed it with the king`s ring, and sent letters by post on horseback, riding on swift steeds that were used in the king`s service, bred of the stud:" — Esther 8:10 (ASV)

Riders on mules, camels and young dromedaries - Most moderns translate this as “riders upon coursers and mules, the offspring of mares;” but the words translated “mules” and “mares” are of very doubtful meaning, since they rarely occur elsewhere. The real meaning of the clause must remain doubtful; perhaps the true translation is, “riders upon coursers of the king’s stud, offspring of high-bred steeds.” So (Esther 8:14).