The winner of the 1973 Triple Crown (The winning of the Kentucky Derby, the Preakness Stakes, and the Belmont Stakes), considered to be one of the greatest racehorses of all time due to his many unbreakable records including the fastest time at the Kentucky Derby and his 31 length win at the Belmont. Also used in reference to a very fast runner who no one can keep with.
Person: Whoa! Slow down Secretariat! I can't keep up!