This could have been a genetic, nutrition or injury problem while the cygnet was in the egg or shortly after hatching. Have you had her eye looked at by a veterinarian to ensure that it is not an infection issue?

In any case, we would recommend that you do not separate the two swans. Keep her as the other swan has already bonded with her and will look after her. If you send her back, there is no guarantee that your other swan will accept another bird or another swan accepts her wherever she is shipped to. So, the best thing for all concerned is to enjoy the swan and know that you may need to provide a little extra care and love for her to keep her safe from predators. This extra love to her and from her will lead you to many years of joy for all of you. The Regal Swan