BTW, there are a number of different species of Equids. Off the top of my head I believe there are, in addition to the common horse, two or three species of zebras, two or three of asses, and also several species of other wild horses.
Yup, and they can still interbreed with no problem. Apparently the extra chromosomes are just duplicates of the basic set so they do not seem to have much influence except in giving a better chance of some of the traits of thoroughbreds being passed on to their progeny. It also should be noted that this is quite unique with horses.