The Petrian Wing took its name from Titus Pomponius Petra, who was probably the original commander of the unit shortly after it was raised from the tribes of central Gaul at the turn of the first century ad. The unit is recorded at Moguntiacum (Mainz, Germany) in 56ad, and is known to have fought for Vitellius in 69 (vide infra).
"... since a single squadron of horse could not protect the broadest part of Italy, he¹ sent in advance infantry, made up of Gauls, Lusitanians, and Britons, and some German detachments with the squadron of Petra's Horse, while he himself delayed a little ..." (Tacitus Histories I.lxx)
"Meantime Claudius Sagitta, prefect of Petra's Horse, by a fortunate voyage, arrived before the centurion Papirius who had been dispatched by Mucianus ..." (Tacitus Histories IV.xlix)
The unit first arrived in Britain in the train of governor Quintus Petillius Cerialis in ad71, and at this time was an ala quingenaria, a nominal five-hundred troopers strong, possibly being initially stationed at Corbridge in Northumberland. The ala was moved to Carlisle sometime during the reign of Trajan (98-117), by which time it had become milliaria, one-thousand strong. They were moved to a new fort nearby at Stanwix when Hadrian's Wall was here replaced in stone (c.ad130), and were stationed there for the remainder of Roman rule in Britain.