An extra first toe, or rudiment of a toe, on the preaxial side of the hallux.
an abnormal extra bone of the foot that grows from the medial border of the navicualr (accessory navicular)
Kidner FC. The prehallux (accessory scaphoid) in its relation to flat-foot. J Bone Joint Surg 1929 : II : 831
The Treatment of Failed Kidner Procedure for Adolescent Prehallux: A Case Report
Park JH, Choi SJ, Ha JM. J Korean Foot Ankle Soc 2007 Dec 11(2):244-247 http://www.komci.org/GSResult.php?RID=0120JKFAS%2F2007.11.2.244&DT=6
Exists in frogs: "In frogs, the first toe is the smallest and the one closest to the body, and it is comparable to our great toe. Technically it is called the hallux, and the small spur at its base is the prehallux. The prehallux is much larger in toads, which use their hind feet in burrowing."