Something bit it when it was smaller, taking out most of the analfin and a sizable chunk of body. However, it recovered because the area where flesh was removed did not contain any organs, just muscle for swimming. Surviving such injury is rare, but it does happen.
It is not going to grow back. Some people feel sorry for such an injured fish, and keep it because they are bleeding hearts who can't stand to see nature take its course. Some simply see it as having a unique look, and derive pleasure from having such a different fish. If the injury is not causing any problems for the fish, it is your choice as to whether you want to keep it. I doubt that it would ever be capable of breeding with that injury, but if it did, the fry would be perfectly normal.
Thank you guys, it has no problems eating or swimming, in fact on my first night of having her she decided to eat my red tail shark. Also from what I have been told by others all my fish seem to be female at the moment so I have/had no intentions of taking on breeding. As long as she is in good health I’m just happy to have her in my tank. Thanks again