One quite common but very special breed of the domestic goat often have a genetic disorder, myotonia congenita, that makes their muscles lock up when they get excited. An evolutionary prank that is kept going due to the practical size of this particular breed of goat, and possibly because of the quite entertaining consequences of a defect like this in large herds. They are called Fainting goats and are so popular that they even have their own annual festival in Tennessee.