Jamieson, Fausset and Brown on Esther 8:10

10. sent ... by posts ... and riders on ... camels, and young dromedaries--The business being very urgent, the swiftest kind of camel would be employed, and so the word in the original denotes the wind-camel. Young dromedaries also are used to carry expresses, being remarkable for the nimbleness and ease of their movements. Animals of this description could convey the new rescript of Ahasuerus over the length and breadth of the Persian empire in time to relieve the unhappy Jews from the ban under which they lay.