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.^[1] It is usually very bad for the kidney and bladder, and only about four survivors were known to be alive as of July 2003.^[2]