So that left me with my imagination and what little I know of the anatomy of both creatures. Of course the biggest issue is things like a heart and the intestines, because if you just shove them together, you get two. Here is how I put them together in my head and subsequently in my book, Druid Derrick.
Heart, digestion, and reproduction is all in the horse part where they belong, albeit with a few alterations which I'll get into later. Up in the human half is all the throaty stuff normally found in the neck of a horse, though sized and adjusted just a little differently too, what with the more upright posture.
So how does the chest and arms work in all this, you ask? Quite simply, really. We all know that the bone structure is there to support muscle, and there is an entire chest and back structure necessary to support arms, even the ribs are important in this, SO they are there. Upper body ribs in a centaur are less important for protecting vital organs so there aren't so many of them and they are wider to support more muscle as well as the bone structure of the shoulders. Shoot a centaur in the human-half heart and you'd be lucky to get past a rib let alone hit an artery, which is there.
That accounts for most of what one might see, but there are other differences smaller and more subtle.
- The hand:
Since I didn't know about the five finger thing with early horses, I went with a four finger structure, giving my centaurs an opposable thumb, but it's not like ours. For us, if someone were to tell you to hold up your primary finger, you'd probably hold up your first finger since it is the most versatile finger on your hand. If you were to ask my centaur to do this, he or she would hold up their middle finger. It is the heaviest and strongest of the three, and their thumb is directly opposite that finger. For us it would be like it was growing out of the middle of your wrist, and no, they cannot set their hand flat on the ground.
Their noses and mouths are very horse in nature, being more molded into their face than ours, and being more closely molded into their lips, which are also very horsey - You wouldn't put lipstick on these lips no matter what. However the mouth is flexible and articulate like ours making speech possible, they just look horsey.
Their hair is also very horsey, as is their ears. They have a mane which gives them the horsey forelocks and it trails clear down their human-half spine until it reaches their horse-half withers.