Belt Hanger - Triangular style.

This device was used on a Roman belt to hang an item.
The item, such as a purse or small knife, hung from the round button or knob on the bottom.
The triangular part was attached to the belt, likely to small hook or loop, though it may have been sewn on.

Such items may also have been used on horse harness straps to hang pendants from.
However, this example might not have been robust enough for harness use.

Circa 1st to mid-2nd century AD.

References:
Fernandez, Roman Equine Harness in Hispania, fig. 2, # 10-11.
Feugère, Gaule méridionale 19, fig. 15, # 116, Augustan era.