主馬 is read as しゅめ (shume) and means equerry.
しゅめ
shume
ばしゅ
owner of a horse (esp. racehorse)
しゅせき
head
しゅよう
chief
しゅけん
sovereignty
しゅにん
person in charge
しゅやく
leading part
しゅえん
starring (in a film, play, etc.)
しゅふ
housewife
しゅかん
one's personal opinion
しゅかく
host and guest
しゅりゅう
(the) mainstream
しゅせん
advocacy of war