To the Arab people, the Arabian horse is called keheilan, which means 'pure blood, through and through'. This is probably the best way to describe an Arabian horse. The true origin of Arabian horse has been lost in the sands of time. It was probably domesticated somewhere in the Middle East and has been it's own separate breed for at least 3500 years. Horses that appear in the Egyptian hieroglyphics(1500 B.C.) look exactly like the Arabians we have today.

Picture From AHA flyer

The Arabian horses originated from a hot arid region, and are for that reason very versatile since the harsh environment allowed only the strongest and the smartest to survive. Arabs are famous for their stamina and endurance. Due to the survival of the fittest, these phenomenal horses, have evolved physically. They have 17 ribs, five lumbar vertebrae, and 16 tail bones, where as other equine breeds have 18 ribs, six lumbar vertebrae and 18 tail bones. Also their arteries and veins lie close to the thin-skinned surface, high tail carriage, and dark skin were a defense against the hot sun and allowed for better heat dissipation.  

References: *The Encyclopedia of Horses and Ponies, Tamsin Pickeral  *Arabian Legends, Marian K. Carpenter