Scientists have previously identified two vestigial toes of horses, derived from the horse's ancient, multi-toed ancestors. However, after close study of one of these ancient ancestors, the 35 million year old "Mesohippus", scientists have identified two small ridges on modern day horse hooves as having evolved from non-fused toes. Dinohippus, a 5-million year-old horse ancestor, provides the missing link of horse mhoof morphology, exhibiting partially fused toe structures. 
"We revisit digit reduction in the horse and propose that all five digits are partially present in the modern adult forelimb. Osteological descriptions of selected tetradactyl, tridactyl and monodactyl equids demonstrate the evolution of the forelimb. Histological, osteological and palaeontological evidence suggest that the Equus distal forelimb is more complex than traditionally conceived. The current understanding is that the horse distal forelimb consists of one complete digit (III) and two reduced splint metacarpals (II and IV). Metacarpals II and IV each exhibit a ventral ridge, which we suggest represents the undifferentiated digits I and V. These ridges are present in the tridactyl Mesohippus, but are absent in the tetradactyl Hyracotherium. The carpal articulations of the five metacarpals match those of pentadactyl taxa. Distally, the frog, a V-shaped structure on the ventral hoof represents digits II and IV, and the wings and hoof cartilages of the distal phalanx are digits I and V. We relate this revised interpretation of the Equus forelimb to Laetoli footprints, and suggest the Hipparion side impressions are created from the hooves of I and V, rather than from II and IV. We show shades of pentadactyly within the Equus manus."
Source:
http://rsos.royalsocietypublishing.org/content/5/1/171782
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