Use 'racehorse' to talk specifically about horses in competitive running sports. That stable is known for breeding champion racehorses.
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Athletic ability
'Racehorse' implies the horse has physical traits and training for speed and endurance on the racetrack. A racehorse needs rigorous training to compete.
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Emotive language
Describing a horse as a 'racehorse' can evoke excitement or admiration for its athleticism. The crowd cheered as the sleek racehorse crossed the finish line.