How do you deal with a young horse that is extremely mouthy or nippy during training?

Extreme mouthiness or nippiness in a young horse during training — persistent reaching for the handler's clothing, equipment, or body with the lips and teeth — is a behavior that needs consistent correction from the first instance, because horses that learn that mouthiness is tolerated will escalate it over time. Clinton Anderson distinguishes between playful foal mouthiness — the normal, explorative behavior of very young horses that is testing the world with their lips — and established nippiness in older horses that has developed into a pattern of targeting humans. Foal mouthiness in horses under six months requires gentle, consistent discouragement rather than harsh correction: a quick, firm bump toward the foal's muzzle or a sharp sound that interrupts the behavior, not punishment that frightens the foal. For older weanlings and yearlings that have developed a confirmed nippiness habit, Anderson's correction is more direct. The moment the horse's head swings toward the handler with intent to nip, the handler steps into the horse's space — toward the horse rather than away — with energy that communicates the behavior is unacceptable. The key is that the handler advances rather than retreats, because retreating confirms to the horse that the nipping movement was effective in moving the human. Warwick Schiller's analysis of nippiness adds the distinction between dominance-based nipping and anxiety-based contact-seeking. Dominant nippers test boundaries and need clear correction. Anxiety-based contact-seekers — horses that mouth and nip because they are nervous and seeking tactile reassurance — need a different response. Correcting an anxious horse's nipping with physical pressure can increase the anxiety and worsen the behavior. For these horses, addressing the underlying anxiety while gently discouraging the nipping is more effective than correction alone. Regardless of the cause, the prevention that works best long-term is never allowing the behavior to go uncorrected even once, because a single successful nip — one that produces no consequence — teaches the horse that the behavior is sometimes acceptable, which is the least consistent and least effective training environment possible.