Polymelia is a birth defect in which an affected individual has more than the usual number of limbs.

Notomelia (from Greek for "back-limb-condition") is polymelia where the extra limb is rooted along or near the midline of the back.

[citation needed] Notomelia has been reported in Angus cattle often enough to be of concern to farmers.

[1] Cephalomelia (from Greek for "head-limb-condition") is polymelia where the extra limb is rooted on the head.

In Greek Mythology: Sleipnir, Odin's horse in Norse mythology, has eight normal horse legs, and is usually depicted with limbs twinned at the shoulder or hip.