sometimes with fish, they can get a bad eye infection which causes it to enlarge and bubble out. If treated in time, the infection can be caught and the eye will go down in size. But if not treated timely or not at all, sometimes the eye will burst. This has happened to some tropical fish that I've had. It also doesn't mean it's fatal, either. Sometimes the eye socket will just heal over and you have a one-eyed fish.

Are these koi in a pond or tank? What sized tank?

They are in a 10 gallon tank. They lived in a pond before I got them. I know that they can't stay in they all together. 4 of them are for someone else. I got them at a swap. They eye socket looks like it has scales in it. There is a little socket but it looks as if there was never an eye. Thanks for the response.