|This article does not cite any references or sources. (March 2011)|
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. In human cases 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.