One day, out of the blue, your sweet-tempered gelding starts acting in unexpected ways—shying at nothing, bucking or rearing, tossing his head—you get the picture. What you do about changes in equine behavior can make a difference in the outcome, so here are some tips.

First, and most important: Always give your horse the benefit of the doubt. He can't tell you when something is wrong, so before you do anything else ask your veterinarian to rule out any health-related problems.

Even the most accommodating of horses might become reluctant to move if he is "tying up," getting arthritic or if his hooves have been trimmed too short, for example. A painful tooth (or teeth that need floating) could cause him to avoid the bridle, toss his feed or rear.

Is your horse suddenly acting "girthy" or bucking a lot? An ouchy back might be to blame. Unexplained spookiness, balking at jumps or a reluctance to travel from light to dark could also indicate vision impairment. General grumpiness could point to ulcers, a low-grade fever or another hidden condition. Mares can become moody when in season, and colts can get aggressive as they mature. Even if the change seems minor, it's worth discussing possible underlying causes with your veterinarian.

If no health issues are found, examine possible environmental causes. Consider your horse's everyday schedule and when undesirable behavior began. Was anything different that day or was there anything new in his surroundings?

A sudden lack of turnout or exercise can cause a horse to act out by kicking at his stall or becoming hard to control—especially if he previously lived outdoors. Also consider any recent changes in the amount or type of feed or hay fed to your horse. Too many or too few calories, an allergy, a sensitivity or even a change in feeding routine could trigger unwanted reactions.

A new or ill-fitting bit or a rider who chokes up on the reins could prompt an otherwise quiet horse to rear. A saddle or girth that hurts is a good excuse for bucking, rearing or bolting (although horses can also buck, rear or bolt if startled, exuberant or bored). And, has your mount had a break from ring work lately? If not, he could well be "ring sour."

Lastly, remember that fear drives many equine behaviors. If a situation triggers memories of trauma or pain, avoidance is only natural. For example, if a normally easy traveler starts refusing to load on the trailer, perhaps he fell while recently being transported. Similarly, a horse that is head-shy for no apparent reason might be recalling previous rough handling whenever he is haltered or bridled.

It can take time and patience to determine what's behind a behavioral change—and some changes are easier to "fix" than others. But regardless, your horse depends on you to understand that something could be bothering him.