Terret

Terrets are commonly made of brass or steel, and they may stand up stiffly, or they may consist of a hinged ring.

[1]: 229 The word terret may also be used for other metal loops for attaching ropes or chains, such as the ring on a dog collar.

[2] Because the terret was often decorated and has the same mounts as harness bells and plumes, sometimes in combination, the word often extends to include these even when the guideline function for the reins is itself missing.

The purpose of bells, for example, was to give audible warning of the approach of a team, as the weight of a load requiring four or more horses in hand makes rapid stopping very difficult.

[1]: 125 Bronze terrets from horse harness are a common archaeological find from the European Iron Age, sometimes with decoration.

Pair of terrets on the harness saddle, and another pair on the neck-strap
Roger rings on the outside of the bridles of these rear horses in a four-in-hand team
The reins going to an individual horse's bit pass through terrets on the saddle and neck-strap. But reins going to horses ahead, pass through rings high on the bridle of the horses behind them.
Fly terrets on the crown of the bridle
Romano-British enamelled bronze harness terret, made in 1st-century Britain, found in France