The Vilna Gaon points out that Mordechai provided these animals to the couriers because he wanted them to hurry. This, despite the fact that they were exhausted from having just traversed the largest nation in the world to deliver Haman’s original decree. Seeing that they were tired, he gave them the fastest possible horses.
The Malbim writes that Mordechai sent the messengers on horses in contrast with Haman. In explanation, R’ Chaim Kanievsky writes that Haman had plenty of time – he had eleven months. Mordechai is in a hurry to save lives.
Interestingly, the Talmud (Megillah 18a) writes that the sages were unsure as to the translation of the couriers’ transportation.
Rashi translates achashtirans as swift camels.
The Ibn Ezra writes that these are a species of mule. After all, the verse says they are bred from ramachs, and the Mishnah (Kilayim 8:5) considers a ramach a mare, mother of a mule. Also, the Arabic word, ramach means mare.
R’ Yosef Kimchi concurs and he adds that achash in Median means large and tiran (misrain) means two. Therefore, the combination of the two words means the mating of two large animals: the horse and the donkey.
R’ Samson Raphael Hirsch (Collected Writings, Volume IV, 286) translates rachash as a draft horse. Parenthetically, he adds that the symbolic meaning of these in TaNaCh indicates a reluctance to listening to one’s master.
R’ Yehoshua Leib Diskin writes that these untranslatable words answer another question from the Talmud (Megillah 3b), which says one must interrupt Torah learning to hear the public reading of Megillas Esther on Purim. This is also brought down as the Halacha (Shulchan Aruch Orach Chaim 687:2). The Halacha (Mishnah Berurah Orach Chaim 690:26) further states from this verse that one fulfills one’s obligation in Hebrew despite not knowing the meaning. But is not Megillas Esther also Torah?! Rav Diskin explains that it is not considered Torah study if one does not understand it. Understanding is an essential component of Torah study. Hearing the reading is still an obligation because persumei nisa (publicizing a miracle) is even greater than Torah study.