Crotal bells (Greek 'crotalon' - castanet or rattle) are various types of small bells or rattles. They were produced in various Pre-Columbian cultures. In Europe they were made from probably before the early Middle Ages and though many founders cast bells of this type, the Robert Wells bell foundry of Aldbourne, Wiltshire produced the largest range. The first medieval designs came in two separate halves into which a metal pea was introduced and the two halves were then soldered or crimped together. Somewhere around 1400 they were cast in a single piece with a ball of metal inside.
Crotal bells, also known as rumble bells, were used on horse-drawn vehicles before motorised vehicles were common. They were often made of bronze with a slot cut down the side. These bells were used to warn other horse-drawn vehicle users (mostly on country roads) that another vehicle was approaching. They came in many sizes, from a small 1-inch version to bells that were many inches across - the older ones were forged while others were cast. They were either hung on a small leather-and-iron harness bracket above the horse's collar on smaller vehicles. On larger vehicles, such as delivery wagons, they were driven into the wooden frame of the wagon.
Crotal bell Wikipedia
A different form of crotal is found in Prehistoric Ireland. The National Museum of Ireland and British Museum have several examples on display dating from the late Bronze Age (800 – 800 BC) which were found in the Dowris Hoard, alongside various brass wind instruments. These are bronze cylinders in the rough shape of a bull's testicle, with a piece of baked clay or a pebble inside. It is presumed they functioned as a type of rattle. The hoard had 48 of them in total, in two sizes. Only two other examples are known, both Irish.