I did something similar in a joking tone
I'd like your opinion ^ ^

[link]

and this


[link]

PI,
Your proportions look pretty good to me, but I have a suggestion and a comment:

Suggestion: A horse's ribcage is deeper vertically than your drawing, which is what probably prompted CKentavr's spouse's comment about the legs being too long.

Comment: There seem to be two "species" of centaurs: the ancient Greek, whose proportions are much like a human/pony amalgam, and whose withers are approximately at the same height as a human's (q.v. the centaurs of the Parthenon frieze); and the more modern species, whose proportions are more human/horse, with withers around 4--5 feet.

I've seen skeletal drawings of mermaids [link] and centaurs [link] on dA before, including a few that try to depict the organs [link] - but I have reservations about calling them "lessons".

My general rule(s) of thumb when drawing centaurs have been simply:

-The horse body should be large enough to at least support a rider (which was mostly driven by a story point), and
-if the proportions generally look equal, and no part seems too large or too small in relation to the other, it'll probably work.

Other than that, as with any fantasy creature, the sky's pretty much the limit.

I've had some interesting discussions regarding the anatomy of fantasy creatures that exhibit human characteristics over the years. Not to present those viewpoints in any sort of scientific or "official" way, but most people I've talked to see creatures like this and tend to "think human". That is, they tend to think of these creatures as two separate halves of a whole (a human stuck to a fish or a horse, for example), rather than a creature that evolved this way. So if a human or horse or fish looks a certain way, their fantasy counterpart surely must share those traits, too. That's where I start to have issues.

For the sake of argument, lets talk about a mermaid. First, define what a mermaid is, and then we'll talk about anatomy. Sounds simple, but it isn't. Over 40+ years, I've seen tons of representations of mermaids. Not all of them fit nicely into the "human upper part/lower fish part" variety. Some are very human, others are very fish-like... and every variation in between. One that I saw years ago depicted a very human-like female that looked "normal" in every regard except she had a fish tail that replaced her hair. Nevertheless, the artist clearly labeled her a "mermaid". Her anatomy is likely going to be significantly different than say, this [link] or this [link] will have. None of them are any more "correct" than any other. I can appreciate their anatomy (whatever it may be) "works" for them without having to wonder if her innards work (or even look) like ours. I don't think they can or even should.

Consider a person born with sirenomelia (or "mermaid syndrome") for a moment. So-called "Wolf people" aside, and while not an actual mermaid, this is about as close a real-world example of what happens if human anatomy deviates a little too much from the norm as I can think of with regards to a having a fantasy-world counterpart. Its usually fatal within a few days because certain organs (particularly the kidneys and the bladder) have abnormal functionality and/or development. So I have issues with the notion one can simply stick a human body to an animal one as-is and expect it to function. But who is to say an actual mermaid's internal organs would be even remotely similar to our own? She may have organs that serve a similar function but which may not look anything their real-world counterparts.

The tl:dr version (and not directed at anyone in particular): don't complain to an artist that "nature would/wouldn't do this" when they draw a mythical creature that has human-like characteristics. I don't care if you have a Phd in biology, biological accuracy isn't why most of us draw them. Are they biologically improbable/impossible? Of course they are, but if I want my mermaids to have larger eyes and pupils and slits for nostrils, I'll do what I can to make them fit in the skull - but don't complain that her face isn't "human" enough looking. I'm not drawing a human stuck to a fish--I'm drawing a mermaid as I envision them; a creature that exists in her own right.

Oh, and I like my centaurs' ears on top of the head -- not on the sides.

</soapbox rant>

Having said all that, I will admit to one bout of critical thinking when I was presented with an explanation of how a mermaid reproduced in a work of fiction. Piers Anthony in Mercycle had characters wonder about this in the text--sometimes openly. Now, the mer-people in the novel were genetically altered humans, but one passage made me wince. I don't have the novel in front of me at the moment, but to paraphrase; the main character observes the mermaid bend at the waist and notes the scales sliding apart and revealing that "it was all there." Which had me wondering what happens when a mermaid and a merman are making out and she suddenly decides to straighten out...