Of course you do! Horses' hearts normally weigh between 10-12lbs, with exceptionally athletic horses' hearts being recorded as 14lbs or larger. That heart has a crucial function: pumping blood around the entire body, which delivers oxygen and nutrients to cells and removes their carbon dioxide and waste products. A horse's heart beats around 28-40 times per minute at rest, but during exercise can beat faster than 200 times per minute. Other things that might make a horse's heart rate increase include excitement, pain, dehydration, anemia (lack of sufficient red blood cells), infection/inflammation, and arrhythmia (abnormal heart rhythm).
One common abnormality of the horse's heart is atrial fibrillation, where due to abnormal electrical activity in the heart the atria (top two heart chambers-see photo) quiver instead of contracting in order to pump blood into the ventricles (bottom two chambers). This greatly increases the heart rate, and can cause decreased athletic performance or even sudden death. Diagnosis includes an electrocardiogram (ECG). Treatment is conversion of the electrical rhythm with a drug called quinidine or with a transvenous electrical cardioversion (TVEC) procedure. However, if the atrial fibrillation was caused by structural abnormalities of the heart, there is a high risk of relapse even if treatment is initially successful.
Other common abnormalities include a heart "murmur". This describes the sound your veterinarian hears in the stethoscope between the normal "lub-dub" sounds of the heart due to turbulent blood flow (blood flowing in the wrong direction). Some murmurs can be due to ageing and thickening of the heart valves, which causes them to be less efficient at preventing backflow of blood when the muscles of the heart chambers contract. Infection and structural abnormalities (such as an abnormal opening between heart chambers or vessels) can also cause a murmur.
The earlier your veterinarian can identify and start treatment of a heart abnormality, the better the chance of success. This makes an annual physical examination a crucial part of maintaining your horse's health.