myriarch
/ˈmɪrɪɑːk/noun
1 Ancient History. A commander of ten thousand men.
2 Asian Hist. Among the Mongols: a military commander of a large force (notionally, a division of 10,000 men). Also (especially in Tibet): the governor of an administrative district under Mongol rule.
Origin
Early 17th century; earliest use found in Henry Cockeram (fl. 1623–1658), lexicographer. From ancient Greek μυριάρχης (Herodotus), μυρίαρχος commander of ten thousand men from μυρίοι ten thousand + -άρχης, -αρχος.