If you forgot your horse anatomy, here's a drawing with some body parts labeled again.
Besides having trouble with the angles of the back legs, many beginning artists tend to have trouble with the slope of the pastern (the area just above the hoof) and the angle of the hoof. Usually they are drawn too straight, as in the image below.
You may actually see a horse like this occasionally in real life but, if you do, that horse has a problem. It is not a normal, healthy leg and horses with this problem usually end up with lameness at some point in their lives. These horses are also not fun to ride. The slope of a normal horse's pastern acts as a shock absorber and softens your ride, so if the horse's pastern lacks that slope, it's a rough ride.
A healthy pastern slope should be at about a 60º angle, and the hoof should be close to the same angle, as shown below.
A beginner artist may also make the mistake of putting too much slope in the pastern, but I don't see that issue so often. My tutorials so far are referring to a horse that is just standing. You could very well see more, or less, slope in the pastern if the horse is in motion. As mentioned, the pastern acts as a shock absorber, so if the horse is in motion the angle can change quite a bit depending on the where the horse is in its stride. If the horse is in a full gallop the pastern can be nearly horizontal to the ground at times, as you can see in the hind leg in my running horse painting here.
It's best to keep things simple when learning. Draw lots of horses just standing for a while. When you get comfortable with that then move on to tackling different leg angles for motion.
Now lets move on to leg length and alignment. The underside of the horse's body tends to slope up toward the rear (unless the horse is fat!) which makes the rear legs look longer than the front legs. This can lead to drawing different parts of the legs at incorrect lengths. Certain parts are actually longer but not as much as we tend to think. Notice in the image below that the fetlock joints are pretty much all in line, the same height off the ground all around. After that the joints start to vary some in height, the rear joints getting progressively higher than the front. The knee joint, you can see, is slightly below the hock joint. Not a lot, though, so keep them close to the same height. The next joints up, the elbow and the stifle, are more noticeably at different heights off the ground. For these joints, position them close to the body line. Drawing a series of horizontal lines can help you get an idea of how things line up.
The guidelines in this and the previous tutorial can be used for most ungulates (hoofed animals) such as cows, deers, and even giraffes. They're not as different as you might think. Let's take a look at the giraffe, for example. Because it has such a sloped back and long neck we tend to think the giraffe's front legs are much longer than its back legs. But, as you can see in the drawing below, the joints of the front and back legs actually line up in a similar way to the horse's legs. It's mainly the giraffe's back that has all the slope.
So, when in doubt, get out the ruler and draw lines and angles across and up and down the body. Compare the lines and angles to the ground as well as other parts of the body. Keep reference images handy. Feel free to print out these images and have them at your drawing table while you learn.
You might also find this book useful for drawing animals in motion: