Iranian tribes such as the Massagetae were believed to be the originator of the class of heavy cavalry known as cataphract. During the time of Achaemenid Persia cavalry was the elite arm of service (as was the case in most civilizations), and many Persian horsemen such as the bodyguard unit of Cyrus the Younger were rather heavily armoured by the standards of the era. By the time of Alexander's invasion cataphract units with both men and beasts being fully encased in armour were already in use by the Persians.