Carving a horse's head out of wood, like any carving, is easy or difficult based on how much detail will be put into it. However, whichever amount of detail the head will eventually take, the process can be made much easier with the application of a step-by-step system or methodology.
Buy the right tools for the job. You don't need an exceptionally large quantity of equipment. To start you can use a sharp carving knife, a utility knife with a large supply of blades and a small hand saw which you'll use to carve your rough shape out.
Find a block of wood that matches the shape and size of your horse head as closely as possible. The closer it is in size and rough shape, the less prep work you'll have to do yourself to form it. Ideally, you'll want to use a soft wood that cuts easily; good kinds of wood include cedar, basswood, aspen and butternut. Get your hands on a piece of wood that is as clean and knot free as possible.
Hew your block of wood into the rough shape of the horse head. Use your saw to cut out the basic block form that will resemble the snout, elongated head, two pointy ears, mane and (if it applies) the neck. You want to make sure that the grain of the wood runs lengthwise for any delicate parts you'll be carving. In the case of the horse head there will be few such parts, so your wood grain can probably go either way.
Use your carving knife to carve the blocky rough shape of the horse head down into a smooth, more detailed outline of the head. For this task you need not concern yourself with the details of the eyes, ears, nose or anything else. Just carve out the well-formed, detail-free shape of the horse's head, ears, mouth and mane. You might want to look at a picture of the animal's head to keep your carving accurate while you work.
Fill in all the details now that you've carved an accurate profile of the animal's head shape and contours. Use your scoop and utility knife to carve out the hollows of the ears. Use the utility knife to carve out the details of the mane hair. Use your utility knife to shape and detail the eyes and mouth. If you haven't already defined the muscular structure of the face and head, do so now with the carving knife. Be careful not to cut away too much in any one place.
Always err on the side of cutting away less. It's easy to fix an error by cutting away a bit more, but much harder to fix a carving error where you've removed too much wood. Use graphite pencils to sketch out the patterns and details you intend to fill in on your carving.