Self-mutilation in horses includes biting, stomping and kicking, rubbing, and lunging into objects. Based on the author's clinical experience, three distinct types of self-mutilation are proposed and described. Type I represents normal behavioral response to continuous or intermittent physical discomfort. Type II, seen in stallions and geldings, can be recognized as self-directed intermale aggression. The behavior includes the elements and order of the natural interactive sequence typical of encounters between two stallions, except that the stallion himself is the target of his intermale behavior. Type III involves a more quiet, often rhythmically repetitive or methodical behavioral sequence of a stereotypy, for example nipping at various areas of the body in a relatively invariant pattern, stomping, or kicking rhythmically against an object. The prevalence of the various types of self-mutilation in horses is not known. In one survey, results suggested that self-mutilation of one form or another has been observed in as many as 2% of domestic stallions. Among and between stallions, self-mutilation varies in frequency and intensity, and can reach levels that are fertility and/or life threatening. Careful evaluation of the horse's behavior is often necessary to distinguish the specific type. Type I self-mutilation, where physical discomfort is the root cause, can be eliminated by relieving the discomfort. For Types II and III, understanding of intermale interactive behavior of horses and the environmental factors that may trigger or exacerbate the self-mutilative form, can be useful in guiding humane management or behavior modification. Pharmacologic interventions may be a useful adjunct to management and nutritional changes.