horse entered in a race; racehorse
Refers specifically to a horse that has been entered or is scheduled to run in a race, not just any racehorse in general.
出走馬のリストが発表された。
The list of horses entered in the race has been announced.
出走馬の中には有力馬も多い。
Among the entered horses, there are many strong contenders.
Compound of 出走 (しゅっそう, 'entry in a race') and 馬 (ば, 'horse').