Adam Clarke on Esther 8:10

On mules, camels, and young dromedaries - What these beasts were is difficult to say. The word רכש rechesh, which we translate mules, signifies a swift chariot horse.

The strange word אחשתרנים achashteranim is probably a Persian word, but perhaps incurably corrupted. The most likely derivation is that of Bochart, from the Persian akhash, huge, large, rough, and aster, a mule; large mules.

The words בני הרמכים beney harammachim, the sons of mares, which we translate dromedaries, are supposed to signify mules, produced between the he ass and the mare, to distinguish them from those produced between the stallion and the ass, But there is really so much confusion about these matters, and so little consent among learned men as to the signification of these words, and even the true knowledge of them is of such little importance, that we may well rest contented with such names as our modern translations have given us. They were, no doubt, the swiftest and hardiest beasts that the city or country could produce.