So, I'm just gonna copy the novel from my last redline, because I'm feeling lazy. xD If you have any specific questions, just ask!
The top line (forehead down to the muzzle) of their head is mostly flat, unless the horse has a dished face (like an Arabian) or a roman nose (like a Draft breed). I, personally, don't draw mouth, because it can make or break a picture. Your mouth was too long and a tad too high. So, when their mouth is empty, their lips aren't drawn back very far. The nostrils were in the rght place, but they should be more defined and larger. (: They have a fleshy chin that leads to a harder, more bony area, in front of their cheek. Their cheek is prominent, flat, and extends up to where their eye is. With the eyes, they are round, but taper at the corners. Whereas a human eye is almond-shaped. Their eye structure comes out of their head a bit, making it hard to draw them flat. Also, the ears need to be smaller. Your ears flared a bit in the middle, which is correct, they were just too long and wide. (:
Where their head attaches to the neck, muscles are very defined. In
this photo, you can see the muscles clearly. There is one long one that runs from just behind the cheek all the way down to the chest. (I don't know names!) I need to edit it in. Also, remember that a horse is mostly neck and barrel. Depending on the breed, legs should be evenly preportioned to the body, with a prominent chest. The muscles on the legs are very easily seen, as are joints. Their knees stick out quite a bit, but not overly bulgy.
The underline starts at the armpit and should be pretty flat, with a little roundness. In certain galloping positions, this area will be completely flat, but when standing it is rounded.
Hind legs are really, really, really complicated. xD They have (depending on the breed!) large butts, to hold all the muscle they need for running. I'd suggest studying photos like these:
x X x XOh! Hair:
Depending on mane length, it almost always lies flat against the neck unless
roached or you're drawing a
fjord. Usually, it's medium-length. Braids are simple to draw, just look at some pictures of different types. Longer manes, such as
friesian or
gypsy vanners are more diffciult, due to the natural curling. Tails are usually below the hock join of the hind leg and start at the
dock.