How to make your horse more realistic

Drawing is an evolutionary process, and the more you practice the better you will get. There are some great tutorials here for color and hair and even shading muscle, but I think if you want to make your horse as realistic as possible, you have to start on the inside then work out to what you actually see on the surface.
When young children, or even some creatively challenged adults draw a horse, they recognize that is has a head, a neck, a body and some legs. While this is true, it can lead to pictures like this:
After some practice, most people will start to realize a horse is made up of segments, and some shading is necessary to make a 3 dimensional object appear to have depth in a 2-D drawing. Since our brains cope best with things they are already familiar with, they tend to simplify these segments into geometric shapes. The jaw, shoulder, barrel and hindquarters are all roundish. The legs are tapered straight bits connected with joints and may have angles depending on the pose.

If your horses are in some phase of this segment stage, stop yourself before you try to highlight muscles in randomly. Think about what a muscle is, and what it does. Muscles are attached to bones for the purpose of moving them. The anatomy of a horse is complex, there are also tendons and ligaments that move or anchor bones, but for a simple art lesson you don’t need to know their names. While you are looking at these horse skeletons, there are a few things that might help you remember them later. First, a horse and a human have almost exactly the same bones in the same places, some are just shaped differently. For instance, the horse’s front “leg” is really his arm. His neck meets his shoulder; there is an upper arm that connects to the elbow. A horse’s knee is the same as your wrist. It is made of lots of little tiny bones put together, which is why they can look lumpy. The cannon, fetlock, pastern and hoof all correlate to the bones in your hand and finger. In the hindquarters, the hip joins the upper leg. A horse’s stifle is the same joint as a human knee, and his hock is your ankle. Notice how the rib cage tapers towards the back and how the tailbones extend at least 8 inches into the actual tail. The head and legs have virtually no muscle, so it is really easy to see the bone structure. The neck, however, is usually so muscular that you can’t see where the bones are at all so I’m going to concentrate on the areas where the bone isn’t obvious, but plays a very important role in what you see from the outside. I’ve circled three points in both the shoulder and the hip. The top of the scapula, the “point of the shoulder” where the scapula meets the humerus and the elbow are the points of interest in the shoulder, while the top of the pelvis, the hip and the stifle form the angle in the hindquarters. Even on the fattest pony, you can usually pick out these points. Once you train your eye to find them, you can draw a line in you mind to connect them and start to see what kind of angles are happening. Different breeds are put together differently; draft horses tend to have upright shoulders, while horses bred for jumping or high knee action have a much more sloping shoulder. Because they are joints, they naturally move quite a bit, and the angle between these points can open and close as the horse moves his legs.
from a great drawing site

To help you start to understand what the bones look like from the outside, I’ve got several pictures of the same horse. First, look at her while she is standing still. See if you can find the points I circled in the skeleton.
If you found them and mentally drew in the dotted line to connect them it might look like this.

Now look at her in motion and see if you can do the same thing. I drew in the first few, the rest are up to you. Pay attention to what happens to the angle if the leg is extended or back, if she is bearing weight on that leg or if it is in the air….Once you program yourself to see these angles, you will be able to start drawing them, and when you do, your horse will take on a more realistic look.

Understanding anatomy will help your art skills no matter what medium you are using. If you want to apply it to digital painting, I'll show you how I do my coats. You are going to have your own unique style, it may be a composite of techniques from other people, or something completely different. If you are just starting, try to do a picture using each of the different styles in this forum and find the one you like best.
For my style, I was never good at coloring inside lines as a kid, and I have no intention of changing now. Most of my original coats I've posted were not based on line art at all, I shaped the horse as I went, but luckily my method works just as well with a base. Here is the one I am starting with.
The first thing I do is add a new layer. I decide on the base color for my horse and use the paint bucket to fill in the entire layer with that color. Then change the layer's opacity so you can see the base underneath.
Next, I'll add some color. It doesn't really matter where you put color, or how rough it looks, or if it stays inside the lines. The dodge/burn tool is ok for finishing touches, but to give my horse a more 3-d look, I really prefer adding colors. Don't just stay in the same palette as your base color. For this gray horse I've added about 5 different shades of gray, but also pink, blue and yellow. This is where you can decide just how shiny you want the horse, which areas are in shadow and start to give him shape. I'm ignoring the mane and tail right now, and coloring in the horse all the way.

Now I break out the smudge tool. I use a bigger round brush to start. This is the fun part where you get to push, pull and swirl the color around to give your horse smooth muscle tone. If one area starts to get washed out, add some more color. You can change the layer's opacity back to 100% so you see how well the colors are blending. Switch to a small smudge brush to refine some of the lines.
Then zoom way in to areas like the head and legs. Use the 1 pixel paint brush and 1 pixel smudge tool to get your details perfect. This is where having a picture for reference comes in really handy.
Once I am totally happy with the body, I'll erase all the extra bits. Set the layer to almost transparent so you can really follow the shape of he base while you erase.

Now hide that layer and add another. If you know you are going to have a background, you may want to pick a color close to your background and paintbucket in the whole layer. Then you are going to block in the mane and tail with one solid color. Add squiggles and horizontal lines of lots of other colors, both lighter and darker.
Take the smudge tool set to any of the irregular brushes. I like different ones depending on the size of the picture and the motion of the tail, but any brush that looks like lots of little specks will work. Start at the top and make long sweeping strokes all the way to the bottom. If you make short strokes just to blend one color with the next, you end up with the raccoon look, but if you go all the way down it will start to look like real hair. Then erase what you don't want for the whole layer. If you didn't fill it in with a color, you won't need to erase anything, but using a color will help the feathered tips blend in with your background. Finally show both layers, and clean up any last details. You can merge the layers, or set the smudge tool to use all layers to make sure the mane and tail look like they belong on that horse.

WHOA that was really really helpful. I think the skeleton connecting on the horses would help the center of balance in the coat. Great job, this is pretty cool.