Dipygus

The dipygus deformity is a severe congenital deformity where the body axis forks left-and-right partway along the torso with the posterior end (pelvis and legs) duplicated. Myrtle Corbin was a dipygus; she married and had 5 children and lived to age 59 or 60. The inner two of the four hindquarters develop much smaller than normal. This is a type of "teras catadidymum" (= "monster twinned below").

Another sort of deformity with extra legs can happen from a degenerated conjoined twin, as may have happened with Frank Lentini with his third leg.