That foot looks awkward
First a note about the above body shape: it would need eyes in the back of its head. Even when this one would descend from merfolk returning to land, there's a design issue. The foot is in the opposite direction i.r.t. the hands. In upward position, it could jump backward very effectively, but in general, animals won't jump in a ...
Unitard, leggings etc.
Close fitting gymnastic clothing is your best starting point
The gymnastic movement they have should be matched by an equivalent style of clothing. It has to move with them, flex and stretch as required, while not dragging on the floor in tripod form.
Of course that's for the common people.
The wealthy and fashionable will ...
This begs the question: don't they have hair?
Surely the bottom part will have hair, just like a horse does. So you'd only need to dress the top part, with a nifty line in riding jackets , longer at the back.
Do your centaur ladies have breasts, or is that taken care of by the horse part? You'd think so, because the reproductive organs are equine too.*
Yes, something very similar was in vogue throughout the middle-ages to far into the renaissance, and occasionally thereafter - even still in modern times during parades in some countries or cultural events: the caparison or horse-trapper:
You might want to adjust the pattern a little, but they're basically fit for centaurs to wear as dresses.
As a ...
They can wear but not at their own
Wearing a dress then zipping and/or buttoning it needs reach of hands and fingers. Centaurs have hands but not long enough to reach all parts of their body so they may not wear anything at their own. They will need assistance for wearing, taking off, changing, trying a dress.
It will be better to leave them as they are. ...
No, Centaurs wear hats
In Greece, a dress would be far too warm for a horse, so the Greek Centaur never wore dresses. However, the beauty ideal among Centaurs did appreciate some decoration, that's the reason why some Centaurs wore a hat. Never heard of any dresses..
You've gotta check old European fashion. During many periods it was common for the richer folk to use pieces of clothing with very exaggerated sizes or features, such as wigs as tall as a child or collars so wide that a moderate gust could flip you over.
In the 1880's very exaggerated shelf bustles were all the rage. This is what I am talking about:
So you ...
I don't see any difficulty with this.
Horse gets tailored suit
Maybe they could hire human tailors to have the work more easily done?
But the suit itself is clearly possible.
In this case the upper part of the legs are left bare to the view. If modesty in your world suggests otherwise a gown could be added around the top part.
To wear it would ...
A wedding train type dress may work.
This type of dress should be easy for a centaur to put on, the long train will flow over her back and down her legs. Hopefully they will look graceful in such a dress although there is a risk it will look like a table cloth ...
This is nothing more than a complete skirt with a bodice attached at the front of the skirt and zippered or buttoned down the back. The piece, when worn, would cover everything you want, but not the horse portion of the Centaur.
BTW, if what you're thinking is, "you know, that's really nothing more than today's backless dress...," you're on the ...
A hole and a zipper.
It might be a little difficult to put on alone, but it's certainly doable. Better yet, have a person assiting you.
Essentially, put a hole in the back of the dress, to fit the horse part. Cut a cut upwards to the hole, and probably strengthen all these seams, since they seem like easy tear points. Then, add a zipper, set of buttons, or ...