A Roman cavalryman's emergency brake

 A cavesson, also known as a Psálion, was a Roman or Thracian cavalryman's emergency brake. 

"Its lower curved bar was connected to a lead rope attached to the saddle or wrapped around the rider's arm. The cavesson presses on the horse's nose, a very sensitive area, and is used for reprimanding a spirited horse, or simply keeping some control of it, when the rider has to let the reins go for fighting. It was also used for leading a horse on foot. On this example, the long angled shanks have a leverage effect increasing the strength of the rider's action on the nose (like today's hackamores)." - Metropolitan Museum of Art

Developed as early as the 5th century BCE, it is described in the equestrian treaty of Xenophon (ca. 430-355 BCE).  Examples have been found throughout western Europe and in Britain as far as Hadrian's wall. Scholars think this distribution was the result of eastern cavalrymen, particularly Thracians, serving in the Roman army.

Romulus supposedly established a cavalry regiment of 300 men called the Celeres ("the Swift Squadron") to act as his personal escort, with each of the three tribes supplying a centuria of men.  The royal cavalry may have been drawn exclusively from the ranks of the Patricians (patricii).  But  the patrician monopoly on the cavalry seems to have ended by around 400 BCE probably due to an increasing demand for trained cavalrymen.  According to Polybius, Roman cavalry was originally unarmoured, wearing only a tunic and armed with a light spear and ox-hide shield which were of low quality and quickly deteriorated in action. This had changed by the Second Punic War.  A stone monument dating to this period shows a rider wearing a variant of a Corinthian helmet and greaves.  Although his body armor is obscured by a small round shield, scholars think he was probably also wearing a bronze breastplate.   A coin of 197 BCE shows a Roman cavalryman in Hellenistic composite cuirass and helmet. Polybius says that by 150 BCE, cavalrymen of the "First Class" were expected to to provide themselves with mail. 

The Jugurthine War is the last war in which Roman citizen cavalry is attested as having played a significant part. After that references to the citizen cavalry become rare and the Roman army seems to have become largely dependent on non-citizen cavalry, either recruited in the subject provinces or supplied by allied kings. As part of the army reforms of Gaius Marius around 107 BCE, citizen legionary cavalry was abolished and entirely replaced by native allied cavalry. The equites had long since become exclusively an officer class (a role they retained throughout the Principate), as the empire had become simply too large and complex for aristocrats to serve as ordinary troopers. At the same time, many of the First Class of commoners had developed major business interests and had little time for military service. Although commoners of the lower classes could, of course, have been recruited and trained as cavalrymen in larger numbers, that must have seemed costly and unnecessary when subject countries such as Gaul, Spain, Thrace and Numidia contained large numbers of excellent native cavalry which could be employed at much lower pay than citizens.

Image: Roman or Thracian cavalryman's cavesson (also known as a Psálion), bronze, 1st -2nd century CE at the Metropolitan Museum of Art.