Instructions
Yell the word "No" at the horse to get its attention. Yell loudly and firmly. Your goal is to get the horses to stop what they are doing and pay attention to you instead. You do not want to approach a horse actively sparring with another, as horses weigh significantly more than humans. You risk injury by attempting to interfere.
Spray the horses with a hose or throw water at them if they do not disengage immediately when you first yell at them. The most important thing at this point is to separate the horses without injuring yourself or them.
Remove the offending horse from the pasture or paddock immediately. You want to remove the more aggressive horse, as other horses in the pasture may be the next target otherwise.
Put the horse in a stall or small paddock by himself. Do not feed the horse or give it any treats immediately after bringing it in, as horses view food as a reward for positive behavior. Check the horse for injuries and treat as necessary.
How to Reprimand a Horse for Sparring
Sparring is a natural activity for horses. It commonly occurs when herd members are playing or trying to establish dominance over one another. Many horse owners do not want their horses to spar with one another because it can involve kicking, biting and potential injury. In some severe cases or with unusually aggressive horses, sparring can even lead to death. Reprimanding your horse for sparring is not guaranteed to be effective in stopping the behavior.