Rearing occurs when a horse or other equine "stands up" on its hind legs with the forelegs off the ground.
It is not uncommon to see stallions rearing in the wild when they fight, while striking at their opponent with their front legs.
A horse that rears when being handled by a human who is on the ground also presents a hazard, as it is able to strike out with its front feet and can also fall even without the weight of a rider to unbalance the animal.
It is a tactic that can be used to dislodge a predator that has landed on the animal's back, it is used when equids fight one another, and a horse can rear slightly to add force when striking out with its front feet.
There are also two movements in classical dressage, the Levade and the Pesade, in which the rider asks the horse to set well back on its hindquarters and raise its front legs off of the ground to varying degrees.
Once on the ground, the rider can prevent further rearing by asking the horse to move, either forward or in circles.
In fact, trained, controlled movements such as the levade and the pesade are deliberately requested by a sophisticated form of collection where a careful, highly balanced rider asks the horse to raise its forequarters by a combination of riding aids that simultaneously gather the horse onto its hindquarters and lighten it in front.
An equine veterinarian can examine the horse's mouth and teeth, back, and feet for possible causes.