This is a very general statement but one that I hear fairly frequently from my clients. My first task is to engage the horse owner in a thorough history, asking as many relevant questions as needed to gather more specific information.

Occasionally, a horse will act in a mysterious or atypical way, a personality change that may be hard to describe. Change in environment, new handlers, riders or management, all can contribute to an apparent change in personality. Physical problems can also manifest as atypical behavior. The behavior might include aggression toward horses or humans, depression, eating less than usual, misbehavior under saddle, or dozens of other variations.

Behavior is mostly controlled by the forebrain. Forebrain disease therefore can cause all sorts of behavioral changes, depending upon the specifics of the condition. Forebrain function can be affected by diseases of the brain itself, toxins in the blood, low blood sugar, blood loss, and liver dysfunction, among many other problems.

Hormonal changes in stallions and mares affect behavior. Pain causes behavioral change too. If there is a possibility that physical issues could contribute to the behavior change, then your vet may be able to help you rule those out. If your vet has ruled out physical causes, and you do not feel confident identifying and changing the behavior, then you may want help from a qualified trainer.