I have a hard time reallly 'seeing' a piece of artwork (that is representational), when it is upside down or sideways. Here, this one I saved and rotated and am reposting: art.jpg
You have an obvioulsy naturally skilled eye for porportion. All those complicated harness pieces, they all seem to be well done. I would not have the patience.
Your eyes and nostrils line up correctly, in a plane in space, as any two objects that are parallel should. This is another area many artists are weak at.
The nostrils are very good on all the drawings.
The thing you want to do, before you start, is think carefully about how you want the head to fit on the page, how you want the negative spaces around it to be shaped, and will they have anything there, any lines to carry the eye through the drawing.
the other thing, when you do portraits, you must find a photo that really 'shows' the character of that animal. If I ownded the horse in the second photo, the one with the gaping mouth and rolled eye, I would not want my hrose to be remembed as such. Of course, you are just practicing, so this isn't /wasn't your intention. But, it is something you ahve to think on.
Photos were the hrose is looking strait at you, as one of your shaggy horses is, do not make the best portraits. you can't really SEE the eye, which is where the soul resides, so a 3/4 angle is better. Looking off at about a 45 degree angly.
As others have mentioned, you don't have enough range of values. The grays are too close to each other in value. You need DARKER DARKS!
Also, since you include EACH and EVERY detail of hte bridle and bit and harness, it can be hard for the viewer to know WHAT to look at. What is most important. So, you may want to draw in more lightly things that don't matter so much, you can experiment with allowing details to fade out, or be 'suggested', or just do something that makes a part of the drawing the focus.
what size are these drawing? what sort of paper, and what kind of pencils? Part of the reason some artists' stuff looks better is becuase they use the right tools. It matters!