It was 37 years after Affirmed before American Pharaoh won the next Triple Crown, and while that probably qualifies him as a "superhorse", the thing to keep in mind about Secretariat is that in winning his Triple Crown 51 years ago, he broke the records in all three races and they all still stand. The second thru eighth fastest times in the Belmont Stakes are all within 0.86 seconds of each other, from 2:26.00 (Easy Goer) and 2:26.86 (Tabasco Cat), while Secretariat is two full seconds faster - an eternity in a horse race, which is why I included his 2:24.0 time in my opening list.
This. The veterinarian who performed Secretariat’s necropsy estimated that his heart weighed about 22 pounds. The average horse’s heart is 8.5 pounds. People talk about Wilt or certain other athletes being freaks, but Secretariat was truly a freak of nature.