Presumably quite early. Several of the surviving commemorative terracottas made after Antiochos' victory over the Galatians c.275-4 show armored elephants. They probably made up at least part of the elephant contingent in every Seleukid field army until after Magnesia. Many of the elephants may have only been hemikataphraktoi, with armor on their legs to protect them from hamstringing, and a thick woolen cloth over their bodies to help fend off missiles.