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?

Or, alternatively,

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.

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    $\begingroup$ @Aify It's not a duplicate. That question specifically says "For this post, we are avoiding the question of how evolutionarily feasible such a creature would be". This question is asking exactly what that post did not. $\endgroup$
    – AngelPray
    Commented Mar 31, 2017 at 23:34
  • $\begingroup$ Sorry, @AngelPray the title was misleading. Perhaps the questions How could a horse or other ungulate naturally evolve humanlike arms? and How might a human develop a second pair of legs should be made more clear. However, after further reviewing this question, It's asking too many things and is definitely too broad. $\endgroup$
    – Aify
    Commented Mar 31, 2017 at 23:37
  • $\begingroup$ The general concensus is it's nearly impossible to evolve extra limbs. Arms or legs is no difference. For six limbs you need to go back to a common ancestor that had still six limbs. This question might alos hold some answers worldbuilding.stackexchange.com/questions/74254/… $\endgroup$
    – Mormacil
    Commented Apr 1, 2017 at 0:05
  • $\begingroup$ This biology.stackexchange answer gives a very good explanation of why such features are unlikely to develop. Addressing the improbability of such evolutionary features would be a good place to start to imagine how such features may develop, but be warned, the groundwork suggests that it is highly unlikely to begin with. $\endgroup$
    – Nolo
    Commented Apr 1, 2017 at 6:14
  • $\begingroup$ The best bet seems to be to start with an arachnid, then make it evolve radically over a half a billion years or so, exo-skeletal features forming inward over time ( decide why so ), soft flesh taking place of chitin ( again decide why so ), and two of the legs diminishing leaving six limbs total. In that case it would seem obvious that four of the legs would be used for walking while two ( or even four for the adventurous soul, preserving a pair, rather than letting them diminish ) would be available for manipulating things in the environment. $\endgroup$
    – Nolo
    Commented Apr 1, 2017 at 6:22