Could a horse be one of the greatest athletes ever?

Good points all round! Was Secretariat fuelled by steroids, or was he a genetic outlier?
There is evidence that steroids were being used in US horse racing by the mid-60s, but I have not read anything about their being used on Secretariat. Steroids have been applied more widely since his racing days, and have been policed all too little; many of them are legal and widely used these days. Recent racehorses are far more likely to be stuffed with performance-enhancing drugs, I suspect, than Secretariat and Sham.
It looks as though Secretariat was an outlier, propelled by that mighty heart and cardiovascular system.
His 50-year records are indeed extraordinary.

HowAndWhy - 2023 10 30

Brilliant and fascinating article.  Never heard of Secretariat, of course, or the Belmont, but had heard of Seabiscuit from the movie about 20 years ago.  The arguments make sense: Secretariat’s huge heart and considerable height gave him advantages, but it is still puzzling that his 1972 record still stands.  The graph of race times speaks volumes: there was a steady fall from 1890 to about 1952, and no discernible improvement since then.  But even if breeding had reached its equestrine limit by 1952, random variation alone would not produce only one Secretariat in fifty years.  Are we missing something?  Steroids, possibly, like the ones that permitted Ben Johnson’s 100m record of 9.79 seconds in 1988?

Peter_G_Moll - 2023 10 27