Polymelia

Polymelia (from Greek πολυ- = "many" plus μέλος (plural μέλεα) = "limb") is a birth defect involving limbs (a type of dysmelia), in which the affected individual has more than the usual number of limbs. In humans and most land-dwelling animals, this means having five or more limbs. The extra limb is most commonly shrunken and/or deformed.

Sometimes an embryo started as conjoined twins, but one twin degenerated completely except for one or more limbs, which end up attached to the other twin.

Sometimes small extra legs between the normal legs are caused by the body axis forking in the dipygus condition.

Frogs in the USA sometimes are affected by polymelia: see Ribeiroia.

In humans

 * In March 2006, a baby boy identified only as Jie-jie was born in Shanghai with a fully formed third arm: he had two full-sized left arms, one ventral to the other. This is the only documented case of a child born with a fully formed supernumerary arm. It is an example of an extra limb on a normal body axis.
 * In the summer of 2005, a baby girl named Destiny was born with a fully formed extra leg in Detroit. This was the result of a conjoined twin scenario.
 * In July 2007 a child was born with four legs at the Lebowakgomo hospital outside Polokwane (South Africa)

In animals

 * A four-legged chicken was born at Brendle Farms in Somerset, Pennsylvania, in 2005. The story was carried on the major TV network news programs and USAToday. The bird was found living normally among the rest of the chickens after 18 months.  She was adopted and named Henrietta by the farm owner's 13 year old daughter, Ashley, who refuses to sell the chicken.  The second (hind) legs are fully formed but non-functional.
 * Four-legged Ducks are occasionally born, such as 'Stumpy', an individual born in February 2007 on a farm in Hampshire, England. See also.