I would recommend finding pictures of horse noses and eyes. (try to vary the pictures so that not all of them are of beautiful, flawless horses. I learned a lot more about the structure and variation of horse heads when I drew from pictures of more "average" horses)
Some good sources are calendars, Google, and bringing your camera to a stable or horse show. Just ask permission before you take pictures of people's horses (some people do get mad if you don't.)
Look at the pictures. Notice what it is about the shapes and curves that makes them so obviously a horse. Spend time finding patterns in the features of horses. For example, the special curve of the nostril or how it changes shape when a horse is breathing hard.
As you are looking, try to draw what you see. It probably wont look much like the picture at first, but if you practice every day you will get much better. Concentrate on the details.
Don't be afraid to draw the head in weird angles and positions to gain a better grasp for the overall shape. Some poses I would try are the eye closed, the eye when looking straight at the horses forehead, the nose with the lip turned up, and flared nostrils such as when a horse is whinnying. Remember to use references for these, wether a real life horse or pictures, because this is how you will make your drawings look more realistic.
This can all take a while, but I would really recommend it. I used to draw circular horse noses and wonder what I was doing wrong, but then I started drawing from pictures and now my horses look very real.
Just remember not to trace, you won't learn nearly as much from that!