In my setting, centaurs are related to other six-limbed creatures such as dragons and griffins (the "fur" is actually made up of tiny feathers). Assuming they were originally creatures similar to six-legged, wingless griffins or dragons, would it make evolutionary sense for them to develop humanoid torsos, arms, and heads?
I imagine it would be possible, though there would be many differences from traditional centaurs (like not having a navel on the humanoid torso).
A real-world counterpart is the praying mantis (picture), where the foremost legs have evolved into something like arms, supported by a thin torso.
If a six-legged walking species evolved to use its front paws as hands, a more upright stature of the front torso would make sense, and you could get something vaguely centaurish. It might have a double stomach, one in each torso, for pre-digestion and digestion. The lungs would probably have to be in the larger 'horse' torso. Or else the 'human' torso would hold lungs, while the 'horse' torso would hold the stomach(s).
If dragons and griffins lay eggs, that would likely mean that your centaurs do, as well (though there are examples of fish giving live birth, even though most fish lay eggs). This would mean that they aren't likely to suckle their young (unless dragons and griffins do; like a platypus, which lays eggs and suckle their young). This means no nipples and no breast on the females.
An issue might be the size of the mouth, both for breathing and eating, but possibly centaurs have larger heads than humans, with broader mouths and bigger nostrils. Also, do they have hoofs, or rather claws like their relatives? Are their eyes reptilian/avian?
You would probably not be able to mistake such a centaur from a horse with an added human torso, but they could be quite centaurish.
I would say, if you do not need them to be directly related to each other, you would have easier time to explain them away.
I do not know the precise details of your world, and whether there are more familiar four-limbed animals there, so I use an alternative Earth as an example.
Say, there is a continent similar to Australia, that become isolated much earlier and much more severely due to whatever cases - geological conditions, change in ocean currents, weird patterns of glaciation. So it became isolated somewhere in the period corresponding to our Devonian period. And the fish that first walked on the shore happened to have three pair of fins instead of two. It could be a suboptimal development, but in the isolated conditions it gave start to the whole plethora of six-limbed land animals.
If the isolation continued through the millions of years, you would most definitely have the cases of parallel evolution and there would be hexapod reptiles, hexapod birds and hexapod mammals. So, your griffins, dragons and centaurs, as well as thousands of other hexapods, are another, isolated offshoot of bony fishes, a clade in the same order as more familiar to us tetrapods.
If, again, the isolation of the continent stopped some time ago, due to natural or man- (or centaur-) made factors, the ecosystems would begin to mix. Some of the hexapods could survive being introduced in the tetrapod environments, and so they would spread beyond their native environments (could be used to explain, why there are only dragons and griffins around, while smaller hexapods didn't fare so well).
Centaurs, being sapient tool users, would generally be better at withstanding evolutionary pressures.
No. Having a torso would require moving your lungs and heart from the body of a horse like creature into a human torso. Not only would you be shrinking your lungs and heart down to a human size, your heart would need to work much harder to pump blood to the far extremities of your body.
You might get away with having a second heart, however since humans breathe through their nose, you're going to have to take huge breaths through a fairly small nose. If you haven't noticed, horses actually have huge nostrils.
However since this is your world, you can do what you want. Using the help of magic or special evolution lines in creatures it is possible to have them distantly linked together. As an extreme example, The scorpions of my world are capable of evolving/devolving into octopuses. It goes something like Scorpion <-> Lobster <-> Crab <-> Octopus. Of course, my world has magic which gives me a lot of wiggle room.
Im going to say no, and here's why hexapedal creatures on other planets/worlds are plausible if not inevitable; that is not up for debate in my opinion. However, I would say that animals on earth and I assume other planets would have body segments that "match-up" one of the main issues I see with most centaurs people design is they are chimerical rather than discrete, that is to say, they look like Frankensteins rather than living breathing beasts. If you create a reptilian or feline-like creature with a simian upper torso up shooting from its "shoulders" you are gonna create something that gets weird looks and enters into a darwin's uncanny valley. Unless your creature resembles a gorilla or chimp bellow the "belt" I wouldn't put a humanoid-human-like form above it. Some people have pulled off some cool alien centaurs one of which is JayRockin-definitely talk to her if you get the chance she may be able to help you more than I can.
If I was going to design a centaur (let's say a bear-like centaur) I would first duplicate either the shoulder girdle or the pelvic girdle of the animal and extend it forward and back, this way I'm working with decent parameters. Then I will edit the forelimbs to my liking while keeping them discrete and so forth. If i were to do internals I would anchor the arms to the false rib cage and should girdle and make the viscera longer and keep most things except for the esophagus, brain, primary heart in the lower body. I could in theory reference some large invertebrate to position organs and edit the external orifices to match. @Klaus Æ. Mogensen rose fantastic points. You will have to divide the viscera with reason. things like water bears, arachnids(scorpions), and mantises are a good reference. I do not know however how your creatures breath, what sort of blood they use, where ears are, or what the basic "hexapod" skeleton looks like in your world; so I may not be able to give you too much feedback. (a sketch i did) https://cdn.discordapp.com/attachments/332688484173217822/582386815189909504/unknown.png http://d.facdn.net/art/jayrockin/1442396298/1442396298.jayrockin_aliencentaur1.png