Refers specifically to a horse bred and trained for racing. Common in horse racing contexts.
He takes care of a retired racehorse.
この競走馬は次のレースで優勝候補だ。
This racehorse is a favorite to win the next race.
Ten racehorses will run in this race.
That ranch raises and trains many racehorses.
That horse retired from racing and became a stud horse.
This foal will probably become a racehorse in the future.