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.
The list of horses entered in the race has been announced.
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').