horses are an off toed ungulate and its part of family Equidae.
That is interesting.  What exactly is an off toed ungulate?
Horse
The horse ("Equus ferus caballus") is one of two extant subspecies of "Equus ferus". It is an odd-toed ungulate mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, "Eohippus", into the large, single-toed animal of today. Humans began domesticating horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Horses in the subspecies "
Odd-toed ungulate
The order includes about 17 species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). 
Despite their very different appearances, they were recognized as related families in the 19th century by the zoologist Richard Owen, who also coined the order name.
Anatomy.
The largest odd-toed ungulates are rhinoceroses, and the extinct "Paraceratherium", a hornless rhino from the Oligocene, is considered one of the largest