Malignant hyperthermia is a muscle condition found mostly in stock horse breeds. The condition is rare - but often the animal appears normal until subjected to either extreme stress or anesthesia. It's most likely seen after the latter.
Then, the heart rate elevates, the body temperature and blood pressure shoot up, the muscles become rigid and, in many cases, the horse dies. It's caused by a gene defect that affects calcium release.
Fortunately, there's now a genetic test for the mutation that causes it, and medication that can be given prior to anesthesia that mitigates the symptoms. They can also use anesthetic agents that are less likely to trigger an attack.