My horse keeps tossing its head in circles what can be done it is not physical already vet checked?

A horse that tosses his head specifically in circles is giving you precise diagnostic information that narrows the cause considerably. If the head tossing were primarily about the bit or general sensitivity, you would expect to see it across all work rather than specifically in circles. The fact that circles are the trigger points toward circle-specific causes. The most common cause of circle-specific head tossing is rein tension that becomes restrictive precisely in the arc of the circle. Many riders unconsciously shorten and fix the inside rein as they go through a circle — the instinct is to hold the bend with the inside hand, but a fixed inside rein creates a block against which the horse braces and eventually tosses his head to escape. The fix is developing a more consistent, elastic contact through the arc and learning to keep the inside rein soft and following rather than fixed and holding. The inside rein's job is to ask for and maintain the bend — not to hold the horse on the arc or pull the horse around the corner. Bend quality through the horse's body is the second major cause. A horse that is stiff through his ribcage and cannot bend uniformly will compensate by bending excessively through his neck — tipping his nose in while his barrel remains straight — and when the rider's inside rein asks for more bend than the horse can produce through his body, the horse tosses his head to escape the conflicting pressure. The fix is more lateral suppling work — leg yields, circles with active inside leg pushing the ribcage out — that develops the body bend making correct rein contact possible. Collection pressure in circles can exceed what the horse has the strength and suppleness to sustain, resulting in the head toss as an escape from a combined demand that is too great for his current capacity. Speed in circles can similarly produce head tossing when the horse's balance and self-carriage have not yet developed to the point where he can maintain correct movement at a higher pace on an arc. The pattern of when in the circle the head tossing occurs is the final diagnostic — consistent head tossing at the same point in the circle points to a contextual anxiety or distraction issue, while head tossing when changing rein direction points to asymmetry between the two directions.