I am honestly surprised no one's done this one yet.
The centaur is a creature from Greek myth with the head, arms, and torso of a human replacing the neck and head of a horse.
Standard Anatomically Correct questions: Could a creature like that exist on Earth? How might it evolve, and what would the result of that evolution look like?
While the ideal centaur would be the classic human-horse hybrid, that might be be bit too far-fetched for real-world evolutionary biology. So, for the purposes of this question, I'll consider a "centaur" to be any creature with an upright humanoid torso (with a head and arms), attached to a horizontal, ungulate-style body supported by four vertical legs. Sapience and dextrous fingers are ideal, but not required.
As this is probably one of the more plausible Anatomically Correct questions (normal horses can easily carry a human rider, after all), this question essentially boils down the the following:
How could a horse or other ungulate naturally evolve humanlike arms?
How might a human develop a second pair of legs, with a result resembling this image [NSFW]? Could the extra legs evolve gradually, and if so, what evolutionary advantages would they give? Or, would it more likely be a freak one-off mutation? Could such a mutant be viable and produce viable offspring? Could it go on to create its own subspecies of centaur-people?
Clarification: This question is primarily concerned with the evolution of centaur-like beings. All the other centaur questions that I have seen or been directed to handwave the evolution and focus on the biology.