Centaurs are wonderful mythical beasts, but what is inside of them is often even more of a mystery.

Some people call them 'humans with horse legs', but I don't think that's true. I think that they have their own structure of organs and systems, plus a unique skeleton that is a hybrid of the human and horse frame.

To begin with, I think that in the humanoid portion a majority of the torso is taken up by the lungs (as seen in yellow), so that they can run high speeds whilst throwing things and carrying heavy objects both on their lower backs and in their arms.

Between/under that is the first heart (Red). I believe there would be two hearts because of how large the body is, and how demanding it would be when galloping. most likely a slightly larger than average heart in the torso, and a fairly big one in the equestrian part of the being.

The large orange-coded organ is the Liver, which is necessarily large in order to produce a very high metabolism. I deduced that this would be essential from a combination that horses graze throughout the day on their food, and that centaurs would double the size of a human and much larger than a simple pony - thus a lot of energy would be required in order to keep the centaur going throughout a working day.

Apart from the second heart, the stomach (light blue), kidneys and bladder (purple) and intestines (dark blue) live in the equestrian part of the 'torso'. There would also be genitalia just below the equestrian pelvis, and in the case of females, there would be breasts over the stomach/intestinal area for ease of young when breastfeeding, as even young centaurs are quite heavy and would not be able to be picked up for long (they develop into toddlers very quickly, and then aging slows to human levels from there.



Now, for the skeleton. The structure would include two ribcages in order to both hold shape of the body and to protect vital organs, however the ribcage in the equestrian torso is narrower.

In order to connect these two 'torso's together, the hips of the humanoid part are more mobile and fuse into the front legs, which are of a horsey shape but human structure with gaps in the thigh bones to allow for mobility and a sleeker frame.
The calves are cut off here as the legs are like normal ones belonging to a horse from the knee down.

As shown here, the spine is very long and curves slowly to avoid issues of strain from meeting at a harsher angle between the two torsos, and dips on the equestrian torso to rise just after the thighs and to the tailbone.