When did cohors equitata milliaria come into existence?

[Steven James]

Nathan wrote:

The regular cohors equitata probably date right back to the formation of the auxilia under Augustus (there are one or two very early inscriptions).

 

I am in 100 percent agreement that the cohors equitata date back to the reign of Augustus, amounting to 600 men consisting of 480 infantry organised into six centuries each of 80 infantry and 120 cavalry organised into 15 centuries each of eight cavalrymen.