Korean radish
Korean radish (mu), shorter and stouter than daikon, often pale green at the top, with firm, juicy, mildly sweet flesh.
Korean radish is sold in Asian grocers, especially Korean ones, and is most reliably found in the colder months. Otherwise, use daikon or white radish from the supermarket.
Substitute:
Daikon or white radish works well, though it is more watery and less sweet, and turns softer when cooked. Some white radishes are also sharper, so taste before using.