I think I can help! I'll edit in just a sec, so I can show you what I mean<3 (it's a long read, so be prepared xD)
Horse full body side profile:
I'm going to show you what I mean with a ref.(All photos by LuDa Stock.DA, I forgot to put it on the pics >>;)
First, take a look at the point of the should and the hip. Draw a circle/dot/mark on these spots.
Do the same for the joints of the knees and hocks.
Now, this takes a tad bit of math/geometry/whatever. You're going to draw a line through each of these points to create an Equilateral Triangle. We want all sides to be relatively the same length, with the line through the leg joints being level. *Note, some breeds like TBs with be a little longer, whereas Arabs will be shorter.
Now that we know where the shoulder, hip, knees, and hocks lie, we can divide the body into circles(get used to this lol)
One for the shoulder/chest, one for the barrel, and one for the hindquarters.
Next, we can draw the neck bones, it's kind of a long, thin triangle. The main line will be the chonky neck muscle, with the topline and throat sort of secondary-lines.
New that we have a neck dorito, let's do the leg muscles, which tend to be ovals and rectangles. I didn't screenshot the process too much, I hope you can kinda see how I did it. I just start at the shoulder/hip and work my way down.
This should show the balance of the horse well enough, now you just apply this to whst you know and put it into your own art style<3
Next Up: The Head(Side Profile):
First, try to look at the head as several shapes instead of one tricky thing. We'll start with the overall shape and build up from there. It tends to be rectangular at its simplest.
Next, we'll add the cheekbone, which is a circle with a flat edge at the top, near the eye.
Next, the muzzle. It's pretty similar to the cheekbone in how we draw it. Pretty circular, then another halfcircle for the chin. Now, the horse I chose has a 'droop snoot', in that the muzzle droops(Which is fairly common in WBs, but not so much in QHs, however many QH have proportianally smaller heads) so it's going to be a bit different.
Now, the jaw muscle/space between cheekbone and mouth. Pretty simple, just an oval.
Now, this part takes a little creativity and knowledge of equine faces. If I could beam the knowledge into you telepathially, I would lol. We're going to refine the muzzle, forehead, and snoot a bit. As you can see below, I changed the snoot a bit to not be so droopy, but It's not terribly important depending on the breed you're replicating.
Now for the eye. It's usually right in the middle of the cheekbone, and up a bit. I've seen people who draw the eyes too close to the ears, or too far down the cheek, so this is pretty important.
That's pretty much how I do it, if you have any questions I'd be more than happy to answer as best I can<3
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I did forget one part, but Show Time added it well. It's just connecting the pieces of the horse together, mainly at the joints.(Shoulder, wither, hip, hock, stifle, etc)