The Newmarket attachment is a simple device that attaches to either side of a pony's bit and allows the pony to be lead via a single line (e.g. lead rope of leash) connected to a ring in the middle of the attachment. Thie then frees up the reins for use by a rider (if the horse is being led while a rider is mounted) and also only requires a single line for leading.
It is seen in the bio-equine world (esp. in racing) where a groom might lead a bio-equine with a lead rope attached to a Newmarket so that a rider mounted on the horse may keep the reins in his hand. A Newmarket attachment is commonly made from nylon, leather, or a metal chain. In the case of thr latter, it is often termed a Newmarket chain.