A scale armour of a heavy cavalryman from the Korean kingdom of Koguryo ca. 6th cent CE in an uknown Korean museum. Note the protection of the neck and also his scale ‘trousers’.   

Below, an infantry helmet and plate armour from the kdm Gaya, ca. 3rd-4th cents CE, in an uknown Korean museum.

Koguryo was a major Korean kdm with eight centuries of existence. Around 676 CE it was destroyed by the allied forces of T’ang China and the Korean kdm of Silla. Its surviving military nobility founded the new kdm B’o Hai in North Korea and Southern Manchuria which was actually a continuation of Koguryo.

Gaya was a relatively small but important tribal union in the southern end of the peninsula. It was finally annexed by Silla.

Below, a map of Korea in 300 CE in which Koguryo is noted as ‘Goguryeo’ (I suppose a more precise spelling by a Korean) and also Gaya in the southern end of the peninsula  (wikimedia commons).