While Japanese traditional instruments boast long histories (up to 1,200 years in some cases, since their importation from the Asian continent) most reached their present forms hundreds of years ago and have not changed since.
The conventional 13-string koto, a zither with moveable bridges, is one that has remained essentially unchanged over the centuries. It is also, however, one of the few Japanese instruments that has enough modified versions in use today to guarantee that they won't go away.
In the 1920s, composer, performer and innovator Michio Miyagi developed the 17-string "bass" koto and the 80-string "piano" koto. The 17-string koto remains one of the most successful of these new instruments, but the koto's 80-string answer to the piano saw no use after its first performance. Other koto variations include the 13-string bass koto (1919), the 15-string koto (1936), the 30-string koto (1953), etc.