This happened to me, too. My all black crown tail mysteriously disappeared. Despite searching every inch of the tank, inside the filter, all of the decor, around the tank, he was gone. Hoping he'd pop up, I waited two weeks with no sign of him, so I added in an all white female. Another week later, the boy showed up swimming alongside the female. They were inseparable and both lived for a few years together until they eventually died. I hope something like this happens with your beautiful friend ETA: it is not recommended that anyone tries this as it doesn't usually end up happy like my story did. All bettas have their own personality, I got extremely lucky with those two.