How could "night rider" be translated into Korean?

 ANSWER: 승야자 (乘夜者)?

According to Merriam Webster, a "night rider" is "a member of a secret band who ride masked at night doing acts of violence for the purpose of punishing or terrorizing," as the Ku Klux Klan (KKK) used to do during Reconstruction in the United States.

The Chinese character 乘 (승) means "to ride," 夜 (야) means "night," and 者 (자) means "a person," so 승야자 (乘夜者) literally means "a riding (乘) the night (夜) person (者)," which can translate as "a person who rides the night" or "night rider."

There is no Korean word 승야자 (乘夜者), as far as I know, but Koreans do use the Sino-Korean word 승야 (乘夜) to mean "under the cover of night" or "under the cover of darkness." Since the pure Korean word for "night" is 밤, and for "to ride" is 타다, 승야 (乘夜) would translate into pure Korean as "밤을 타서." Also, the Korean expression 틈을 타다 literally means "to ride a crack or opening," but translates as "to seize an opportunity."

By the way, the word 승용차 (乘用車) literally means "a riding (乘) use (用) car (車)," which can translate as "a passenger car." And 승용마 (乘用馬) translates as "a horse used for riding," "a riding horse," or "a saddle horse."

From Dong-A's Prime Korean-English Dictionary

From Dong-A's Prime Korean-English Dictionary