In ancient times, messengers delivered messages on horseback.
Tips
usage
使者 has a formal or literary feel. In modern usage, it often appears in figurative expressions: 和平使者 (peace messenger), 友谊使者 (friendship envoy), 文化使者 (cultural ambassador).
memory
使 (send) + 者 (person) = the person who is sent. The suffix 者 works like English '-er': 读者 (reader), 作者 (author), 使者 (messenger).