The age of a horse can be roughly determined by examining its teeth. Evidence that people knew this long before 400 A.D. is witnessed by the appearance of the above expression in the writings of St. Jerome, who called it a familiar proverb at the time. Don't look a gift horse in the mouth, which means accept presents gracefully without trying to find something wrong with them or determine how much they're worth, is literally reproduced in German, French, and other languages, though its first English use is don't look a given horse in the mouth. A variation the phrase is always look a gift horse in the mouth - examine gifts closely, there may be strings attached. Cynical, but consider the Trojan horse.