A horse needs big lungs to run that fast
This pink, slimy, colossal thing laying on the table is a pair of horse lungs. It seems impossible to believe that a simple horse would have such massive organs. However, most of these animals race or carry heavyweight.
Physical activities like those require the ability to inhale and exhale large amounts of air. It is only fair that well-built, fast horses will have large lungs. They should also have a big heart able to power up their whole system.