There is more to decoding your horse's body language than you may think.
If a horse's ears are back, stay away. This is one of the first tips newbie riders are given. It may be a good rule for beginners, but like most things, horses aren't black and white and can't be simplified quite that easily. Their ears, tails, heads, and legs can tell all sorts of stories, and like humans, their moods and focus are constantly changing.
Paying attention to all the nuances of your horse's body language, will not only teach you a lot about your individual horse, it will make you a better horse person overall.
The ears and their many positions.
If your horse's ears are swiveled forward it means that he is attentive, listening to something in front of him. Alternatively, a horse whose ears are swiveled back is listening to something behind him.
You may also encounter a horse whose ears are swiveling rapidly in all directions. This indicates a horse in a heightened state of anxiety, trying to locate the source of a sound. Ears pinned flat against a horse's neck is an obvious indicator of an angry horse.
Always handle anxious and angry horses with caution.
Head positions and their meanings.
When horses raise their heads it means that they are alert, focusing on something in the distance. They are probably about to move in excitement or anxiety, and head raising is often combined with or followed by rapidly swiveling ears.
A horse whose head is lowered is relaxed, or focusing on an object very near to them.
Tail positions have meaning, too.
A horse that is excited will "flag" his tail, raising it up and carrying it above his body. While many people think this applies only to breeds like Arabians, any horse will do this if excited enough.
On the other hand, a horse that clamps his tail down is nervous or stressed. If your horse does this when ridden it could be a sign of pain and you may want to consult your veterinarian.
Even leg positions can mean something.
A horse that is standing with a hind leg cocked -- toe touching the ground and hip dropped -- is a relaxed horse. Horses may occasionally shift their weight to the opposite leg, just as humans sometimes do when standing.
If your horse is shifting weight rapidly, it may be indicative of pain and you may need to call your veterinarian or farrier, but generally horses that stand this way are simply relaxed and taking a load off.
Your horse's eyes and nostrils are good indicators of nervousness and alertness as well.
The more attention you pay the more you will find your horse speaking to you simply through his body language.