A. It is difficult to say much about what brought on a sudden attack of this type in a 14-year-old stallion, assuming he has not shown similar aggressive tendencies in the past. An attack of this type can have both physical/medical as well as social and handling causes. In rare instances, something as simple as a sudden startle can evoke fear sufficient to provoke a sudden severe outburst, but the owner likely would have recognized a tendency for that type of reaction before this age.
So first, I would recommend a detailed veterinary workup including a neurologic exam. Some medical conditions can affect the brain and should be considered when uncharacteristic and severe aggression occurs. For example, liver disease can affect the brain enough to cause aggressive outbursts. Also, a sudden sharp pain or startling stimulus (e.g., static electricity spark) can elicit such a response.
Another common cause in stallions and geldings is what we call displaced social aggression. When a horse is threatened by another horse, but not at liberty to attack that animal, he might lash out at a person or another animal that is easily within reach, especially if positioned between him and the other horse. This reaction is not widely understood, but it’s important in such cases to appreciate that gelding a stallion will not necessarily reduce the likelihood of further attacks. In fact, if this is socially provoked displaced aggression, triggered when the male is threatened by other males, gelding can result in an even greater insecurity and, so, a lower threshold for intimidation by other horses and for lashing out.
One of the most serious attacks of what clearly looked like displaced aggression that I have personally witnessed involved a pony gelding being led near the pasture of a threatening stallion. It all happened so quickly, but a second before grasping the handler’s arm, the gelding clearly showed signs of trying to submit to the threatening stallion. I have seen many cases of a quick nip or grab at clothing, but in this case the pony’s open-mouth lunge and hold onto the handler’s bare wrist resulted in considerable soft tissue damage. As is so often described, the little fellow was immediately docile, as if he didn’t know what had happened.