Polymelia
This is what happens when having a twin can go wrong. Sometimes twins can fuse with each other’s parts and create deformity at birth. One of them is called polymelia and it is thought to be created when a twin is absorbed with a conjoined twin. It can appear in different ways, but usually, it is when they have an extra limb.
The limb in most cases is useless and just hangs off the body, it can be surgically removed depending on how deeply it is attached to the rest of the body. In some cases, doctors can not remove the extra limb because it could risk the life of the patient. The deformity also can occur in animals.