Confucius said: “A fine horse is admired not for its strength but its heart.”
The character 德 (dé) is usually translated as “virtue”, which would also be fine in this context. “Virtues” or “merits” denoting the horse’s temperament, obedience, and spirit would also be good alternatives to “heart”.
According to some commentators, 驥 (jì) was originally the name of a famous horse that could reputedly run a thousand miles. Of course, a thousand miles could simply mean a long distance.