Sirenomelia

Sirenomelia, also known as Mermaid Syndrome, is a very rare disorder in which the legs are stuck together, making them look like a mermaid's tail. This, however, is as rare as conjoined twins, and only happens to one out of every 100,000 live births. It is usually very bad for the kidney and bladder, and only about four survivors were known to be alive as of July 2003.