mounted warrior; cavalier; knight
Refers to a warrior on horseback, especially in historical Japanese settings. The English glosses 'cavalier' and 'knight' are approximate; the term is culturally specific to Japanese mounted warriors.
Mounted warriors of the Warring States period fought using their mobility.
I saw the armor of a mounted warrior at the museum.
騎馬武者 is the clearer modern term for a mounted Japanese warrior, while 騎士 in this historical sense is a more literary or older way to say the same thing.
Compound of 騎馬 (kiba, 'horseback riding') and 武者 (musha, 'warrior'). The term is straightforward and describes a warrior who fights on horseback.