This isn't a question about an anatomically correct centaur and its features, but I will assume general characteristics of a centaur and the objective is to find what environment would allow an ancestral hexapod to evolve until acquiring said characteristics. This is a way to explain the presence of centaurs.
I will not focus on the absolute origin of these hexapods.
This question arises from the fact that I was unable to think about an adaptive motif to an environment in which a centauroid creature could evolve.
Considering the principal features, humanoid torso over a horselike body. A horselike body indicates highly cursorial habits, with massive lungs to permit powerful races and unguligrade legs with just one hoof, while the humanoid torso has an erected section of the spine exactly over which the horse's neck and head would be, having two long arms with a high mobility range and manipulator hands on them, indicating arboreal habits or that he has to stretch its arms over its head to reach its food.
What I mean by this is that it seems that a creature as similar as possible to a centaur could not evolve naturally, without magic and without things like genetic modifications, since its characteristics seem mutually exclusive or only useful in very different environments (possibly jungle and plains, respectively).
I have tried to find my own justifications but I am not really convinced. For example, a herbivorous animal that needs to evolve in order to reach its food in the crowns of the not-so-scarce but not-so-abundant trees in a savanna; maybe this creature would need to search on the trees, but in this case it's more probable for it to just evolve as a hexapod giraffe, just lengthening its neck.
So the question is, what environmental pressures will lead an ancestral hexapod to evolve into a "centauroid"?
To consider, the ancestral hexapod could be just a primitive mammal but with six limbs, but at the discretion of whoever provides an answer, this creature could change if it considers it so.