Courageous Heart and Valiant Heart are conjoined twins. They were born with Courage’s left side and Val’s right side’s completely fused together. Their front arms were deformed and stuck together and neither of them had a back leg.

They stayed together for a long time before their parents were finally ready to go ahead and attempt the operation to get them separated.

It was a very risky operation, and no one was sure the twins would make it. They had internal organs twisted together and bones fused. Being of the royal family, they had the best doctors in Equestria performing the operation. Eventually, they were successfully separated and now the twins are happy two legged foals. (If your wondering they have custom prosthetic legs being made for them.)