How big is a horse's heart?

Most horses hearts are about 8 to 8.5 pounds, or less then 1% of their body weight. Some breeds have heavier/ larger hearts.

Arabians have a heart around 7.6 to 8 pounds or 0.76% of their body weight.

A draft horse's heart is smaller by proportion and is about 0.6& of their body weight, so a 2,000 pound draft would have a 12 pound heart.

Thoroughbreds tend to have heavier/ larger hearts on average with the typical one weighing about 10 pounds, though some famous Thoroughbreds have been found to have relatively heavy/ large hearts. Secretariats heart was about 22 pounds, Shams was 18 pounds, Eclipse's was 14 pounds, Phar Lap's heart was also 14 pounds.

About the size of a football.... About 10-12 pounds :)