The name Perissodactyla means "odd-toed." This group of ungulates includes horses, tapirs, and rhinos. The name of their order derives from the fact that their middle toe is larger than the others, and the plane of symmetry of the foot passes through it, a condition called mesaxonic. Most species have three digits on the hindfoot and three or four on the forefoot, but in some only a single digit, the third, remains. Some species have horns, but these are dermal structures without bony cores, and they are located on the nasals or frontals in the midline of the skull.