Korean yen
The Korean yen was the currency of Korea between 1910 and 1945. It was equivalent to the Japanese yen and consisted of Japanese currency and banknotes issued specifically for Korea. The yen was subdivided into 100 sen. It replaced the Korean won at par and was replaced by the South Korean won and the North Korean won at par.
Korean currency
The country then had to switch over to a new currency unit named "hwan" replacing won at 1 hwan = 100 won. Like South Korean won, this currency unit was also issued in banknotes initially but in 1959, coins denominated in hwan were also issued and were the first circulating coins in South Korea. Hwan had a peg with US dollar but with time it also got devaluated. This was the reason for the reintroduction of won in 1962 as the official currency unit. South Korean currency was still pegged to the US dollar until in 1997 when it was floated into the world market.