I always found this interesting:
“A necropsy revealed his heart was significantly larger than that of an ordinary horse.[28] An extremely large heart is a trait that occasionally occurs in Thoroughbreds, linked to a genetic condition passed down via the dam line, known as the “x-factor”.[28][30][31][32] The x-factor can be traced to the historic racehorse Eclipse, which was necropsied after his death in 1789. Because Eclipse’s heart appeared to be much larger than other horses, it was weighed, and found to be 14 pounds (6.4 kg), almost twice the normal weight. Eclipse is believed to have passed the trait on via his daughters, and pedigree research verified that Secretariat traces in his dam line to a daughter of Eclipse.[28] In the 20th century, the heart of Phar Lap was weighed and also documented to be 6.35 kilograms (14.0 lb),[33] or essentially the same size as that of Eclipse.
At the time of Secretariat’s death, the veterinarian who performed the necropsy, Dr. Thomas Swerczek, head pathologist at the University of Kentucky, did not weigh Secretariat’s heart, but stated, “We just stood there in stunned silence. We couldn’t believe it. The heart was perfect. There were no problems with it. It was just this huge engine.”[29] Later, Swerczek also performed a necropsy on Sham, who died in 1993. Swerczek did weigh Sham’s heart, and it was 18 pounds (8.2 kg). Based on Sham’s measurement, and having necropsied both horses, he estimated Secretariat’s heart probably weighed 22 pounds (10.0 kg),[28] or about two-and-three-quarters times as large as that of the average horse.
(Source-Wikipedia so take with a grain of salt - but I’ve seen other sources that confirm this information.)
Re the post above - Sham was a half cousin of Secretariat and was also a great racehorse, but always overshadowed by Big Red.