The Praetorians used the oval shield early in their history as is guaranteed by the tombstone from Aquileia. They were in continuous use into the reign of Domitian (AD 81-96). This has been taken to contrast with the rectangular scutum adopted universally by the legions in the first half of the 1st Century. By the end of the 2nd century, both public and private monuments show a return to the oval shield for both Legionaries and Praetorians, although a 3rd Century example of Dura Eoropos proves that the rectangular scutum did survive.
AH6710 Praetorian Scutum