Giving birth is anything but predictable, no matter the species. While the incidence of twins is not uncommon in humans, twin horses are a real rarity. Female horses are classified as monocotous, meaning that they are only able to support one fetus. Becoming pregnant with twins can be uncomfortable and dangerous for the mare and foals. It's estimated that only 9 percent of twin pregnancies survive to term, with only 14.5 percent of those resulting in two live foals. According to Dr. Mina Davies Morel, head of the Equine Studies group at Aberystwyth University, twinning in horses has increased with the increase in monitored breeding practices. Because of this, heavily bred horses like Thoroughbreds and Arabians are more likely to conceive twins.