Saint Demetrius, Byzantine Steatite Icon

Saint on horseback. Demetrius with a sword and a padded cuirass. Steatite (soapstone), Kremlin, Moscow.
Source: protostrator. Soapstone Warriors (military saints in byzantine carved icons)

Fig. 51. St Demetrios on horseback, 13th/14th-century steatite icon; Armoury of the Moscow Kremlin.
Source: Arms and Armour of the Warrior Saints (Medieval Mediterranean)

Referenced on pp8-9, MAA 89 Byzantine Armies 886-1118 by Ian Heath & Angus McBride:
Though Byzantine artists do not seem to have normally considered soldiers as suitable subjects they nevertheless drew, carved and painted endless numbers of military saints. This is one of them, St Demetrius, from an 11th-century ikon. He carries a large circular shield and wears quilted body-armour, probably an epilorikion. Note also his horse’s tied tail. A helmet would normally be worn, but saints are usually bareheaded in Byzantine art.

See also Saint George on a Steatite Icon, Vatopedi monastery, Athos, Greece, 11th Century
Other 11th Century Illustrations of Costume & Soldiers
Byzantine Illustrations of Costume & Soldiers