Onibi

Onibi is an enemy from Ninja-Kun. Onibi only appears if you remain on one level of the stage for too long a period of time. Once it appears, it drifts back and forth on the screen, homing in on your level. It will never go away, and it can never be defeated. Therefore, in order to avoid having to deal with this nuisance, keep moving. Even if you are strategically waiting in a particular position for an enemy to fall, make sure that you jump up or fall down every once in a while to reset the time that it takes for Onibi to appear. Once it shows up, all you can do is avoid it.