This is a part of the Anatomically Correct series.
If a magic-free fantasy/SF world were to have naturally evolved hexapods as humanoid as possible, living among bipedal humanoids, how would they actually look, live and behave like?
- Fur, feathers or naked skin with some hair (pubic, head/mane, chest, tail)? Clothes and armor?
- Feet with (even/odd) toes, (split) hooves or paws with claws? Knees?
- Short or long, fancy or usable or stabilizing, bushy or skinny tail?
- Head and brain size, also other organs (e.g. for breathing)?
- Horns or antlers? – Cows and deers are roughly the same size as mules, ponies and horses, but this is also of interest for 2-legged goat and cattle chimeras (i.e. faun and minotaur).
- Movable flat long hairy ears?
- Grass, leaves, fruit, meat or mixed diet (hence teeth and mouth)?
- Size, capabilities and developmental status at birth?
- Age and duration of infancy, childhood, puberty, adolescence, adulthood and life expectancy?
There are several six-limbed mythical creatures (hexapods) with human-like intelligence or at least typical features of mammals, e.g. quadrupedal centaurs or winged angels, dragons and griffons, maybe also harpies in Mediterranean/European mythology. Like other fantastical creatures, e.g. sphinx, minotaur and faun, they are mostly described and depicted as blends (chimeras) of known animals and usually a human part, but that doesn’t have to be taken literally. For an example of an unusual, because very human-like centaur, see this NSFW picture. Shoops like that are probably easy to make for savvy people, but so are “humanimal” morphs. They may be aesthetically pleasing, but not very realistic.
There are no hexapod mammals in reality obviously, whereas 6 (and more) equal legs (or tentacles etc.) are normal in insects and some water-based animals (crustaceans, octopuses, squids) and some have developed further their back limbs (e.g. grasshopper, crickets) or front limbs (e.g. mantis) or at least one of them (e.g. some crabs).
You cannot simply put a human torso onto a horse body to get a centaur as several questions here have shown, so I think of horse-like as meaning “the approximate size and number of legs of a horse” and of human-like as “able to think and converse and use manual tools”.
- Ideal centaurs: 1 torso and 4 legs
- Anatomically correct: griffins
- Anatomically correct: angels
- How could dragons be explained without magic?
From the nipples question, I assume 2 teats at the frontal crotch or slightly above. The genitals are probably at the rear end, as is the anus. Grazing may be possible, but cannot be the main source of nutrition; an omnivore diet and digestive system seems plausible although not necessary. I like the navel nostril idea mentioned in an answer to the oxygen question, but anything else would look too strange, so probably big noses or big heads over all and probably a short muscular and flexible torso.
Wings are hard to manage for humanoid bodies as they are and I think even more so if separate from arms/hands, so I would rule out winged hexapods altogether.
It’s not at all realistic for centaurs etc. to be able to interbreed with humans or the animals they’re looking similar to, but interspecies sex (or rape) is probably inevitable and the topic of much folklore.