Take a look at Alexander the Great’s horse, he was one such animal. His name was Bucephalus which translates as Oxhead. Some accounts say that he was so named because he had the image of a bull's head on his shoulder. The name conjures up mental images of a warhorse with a massive head and chest, that carried brazen young conqueror. Quite the picture, but did you know that Bucephalus had vestigial toes (extra, non-functioning).
Toes on a horse are an evolutionary legacy akin to human tailbones and gills in womb. Embryos of modern horses develop the rudiments for three toes in utero. Ordinarily, the middle toe will eventually outgrow the outer ones which then become splint bones. This central toe is then the one which will support the horse through contact with the ground, ie the hoof. Bucephalus’was a polydactyl, which means having more than one toe; his toes did not develop in the normal manner during gestation.
The legend of Bucephalus’ mutation was nurtured, imbuing the horse and his owner with mythical properties. The toes, in some way, embodied the outstanding nature of his master’s life, as if everything he touched was extraordinary. Centuries later Julius Caesar hung on the coattails of this myth by sourcing a three-toed horse and protesting that it would let none other than the man himself ride it.