I think he was a twin too. In humans they would have called that a fetus en fetu. It means that there was twins but one grew faster than the other and actually grew around the other embryo. A 9yr old boy in India looked like he was pregnant; he had his twin surgically removed from his abdomen. It looked like a partially formed fetus, had lots of hair and one could see fingers. Your chick likely developed partially around his twin and only the legs continued to develop. I've seen snakes and cattle in a similar fashion (on TV). Someone here had a 3-legged chick but it didn't make it. I think maybe there was more developed from its twin on the inside of the chick and its little heart couldn't handle the stress. Be watchful.