iKoto

Wow! This is a cute and smart toy. You can actually play koto on your iPhone and iPad. 
It comes with the basic sets of tuning. You can also save your own tuning.
Under the setting menu, there are oshide, hikiiro, pizziacato and sukuizume. You can even tilt the koto to get a comfortable angle to play with. You can also record while you play.
You can download it from App Store. It's not free but very useful for a reasonable price. Not as good as the Apple version but there's a Galaxy version, too.
Have fun!