The Roman cavalry was certainly not the primary weapon of the Roman army, which heavily relied on its infantry to win battles and wars. However, the cavalry provided several vital services which were essential for expanding Rome's territories.
The Roman cavalry's functions on the battlefield included:
During the Roman Kingdom and much of the Roman Republic (753 BCE to 107 BCE), each soldier had to provide his own equipment. That meant only the richest of Rome's citizens were cavalrymen as they would have to provide their own horse and it's upkeep. During this period, the cavalry was lightly armored. It had high mobility allowing to be moved around the battlefield very quickly.
As the Roman Republic progressed, the cavalry became better equipped:
After the Marian reforms (107 BCE) and the establishment of the legions, each legion had 300 cavalrymen. These were divided into ten squadrons of 30 cavalrymen each.
Roman Auxiliary Cavalry (alae)
The Roman auxilia was established in around 30 BCE, much of Rome's cavalry forces would be recruited from the auxiliary's ranks. The 'alae' were the cavalry units of the auxilia. The auxilia were usually organized into units of 500 cavalrymen commanded by a Roman born prefect. Double strength units had a total of 800 riders. There were also part cavalry part infantry auxilia units (600 soldiers in total: 480 infantrymen and 120 cavalrymen.
The alae were specialised units so not all units were identical. Much of the auxilia cavalry was equipped with mail armor similar to the cavalry of the Roman legions. However, other units such as the 'equites cataphractarii' were very heavy shock cavalry. However, there were also lightly armored cavalry units which could be moved around the battlefield quickly.
Read more about the Roman auxiliary's cavalry units
Importance of Horses
Beyond the battlefield, horses were of importance for a variety of reasons.
Cavalry horses also placed a large strain on an army's resources. They required large amounts of food and water; consuming a kilo of barley and twenty-five liters of water each day.
We can also measure the effects of selective breeding during the Roman Republic and Roman Empire. The army obviously wanted the tallest and strongest horses for their cavalry. During the start of the Roman Empire, the average height of a horse was 120 cm, towards the end of the Roman Empire horses were as tall as 150 cm in some regions. An increase of 25% over several centuries.
The average life expectancy of horses belonging to the Roman cavalry was only around six to ten years old.