I had difficulties creating a realistic horse drawing until I studied the human anatomy, in great details, to find out the similarities between the two muscular systems. In making such links, learning how to draw a horse became much easier easier.
Despite the obvious differences in bone and muscle structures between a human and a horse, the location of key bones and muscles remain the same for both. The task is to pinpoint these sections.
For example, those extra 'reverse bends' in the horse's front arms and rear legs are actually wrist and ankle joints. Realizing this, you can quickly understand that the forearms, quads, elbows, knees, etc, have been shortened and shifted up into the body.
To that extent, it is highly recommended to compare a few connections between the horse's muscles with the location of the human muscle counterparts through established anatomy sketches you can find anywhere.
Always start off any drawing with simple sketches to outline the composition and form. Think about the ribcage and the neck. How will you attach the shoulder blades to the arms? What type of depth and shape does it make on your frame?
Compared to the human body, the neck region is drawn in a similar manner. For the arms, draw flow points that signify where the deltoids, biceps, triceps, etc, are located at. The same goes for the legs.
Also, in each particular joint for the wrists, ankles, and hooves, add bulges of bone and ligament since horses are really lean in those areas. For the face, look at budges around the eyes, nostril regions, and especially the chin connecting to the neck.
Even the neck will have similar drawing characteristics to the human neck as you suggest the muscles running from the ears to the collar bones. Once that is done, create a new layer and fill it in. Afterwards, lock that layer and slowly work on the initial shading.
In this step of a realistic horse drawing, we are going to switch to the smudge tool. Select either a hard brush or a soft round brush. It does not matter because we are going to set the sensitivity to change the pressure of the brush.
From here, smudge softly and carefully in long drawn out strokes in the same direction of the muscle groups. For example, smudge the neck muscles down to the collar bone, and then smudge the collar bones up to connect to the shoulders.
Add in additional shadows like the belly region and the inner limbs while smudging upwards to create some rib muscles. Make sure to keep track of the joints as the kneecaps/quads and elbows/biceps will be very close to the lower side of the belly.
For the rest of this step, switch over to the eraser tool and gently define the outline further. Your main focus would be the joints. Erase or draw into the long limbs so the bulges are showing. Of course, erase any extra gesture lines that are out of place as well.
You will be locking down your horse object often to draw within the confines of the layer. Start by switching to a smaller chalk brush to work on head to define the nostrils, eyes, mouth, and ears. Use a larger chalk brush to refine additional muscles.
From there, you can also start working on the mane. Use short bursts to create the flowing hair and then smudge it gently into place. You can do this for the tail as well. Repeat until you get a desired effect.
For the legs and arms, paint little bulges of bone near the joints to give it more depth. You may want to create addition flow points of muscles here (to further refine the muscle mass) and smudge it into place.
Once all these are done, begin refining the inner limbs with simple cast shadows. Unlock the layer if you wish to smudge the tail and mane a bit more. Otherwise, use this opportunity to clean up the outline even further.
The entire drawing can be a bit dark as you will have no lighting elements attached to it. Adding realism requires adding some light sources in. Lock down the layer, swap to a round brush, and gently go over the back of the animal.
There are also rim light to consider. This is best handled by a small chalk brush where you simple draw a light outline very carefully across the borders of the animal. Photoshop can add outlines to an object but it can be a bit artificial. Be wary of this.
Additionally, you can add in more details like veins or extra tendons using this small chalk brush to give even finer details. After that, consider adding a simple cast shadow on a different layer to give the animal some weight.
While the overall picture should be done at this point, you can further increase the contrast through any global filters/effects. Some artists like to do it right on the horse drawing while others like to give contrast on different layers to preserve the original drawing.
The same can be said for any sharpening filters. These will sharpen the entire drawing to bring out even more details. Again, it can either be done on the original drawing if you are sure of it or on another layer if you wish to continue playing around with it.
There is definitely a lot of freedom and options available. Keep experimenting to find out what is best for your work flow. I hope you found this realistic horse drawing guide insightful as I had in making it.