A hair. Looks sharp enough that it could be on the film side of the shutter in which case getting it out should be easy. If it is on the other side of the shutter and your camera has an interchangeable lens, you should be able to get at it with removing the lens and putting up the mirror. If you don’t have an interchangeable lens, it might require convincing the camera to open shutter while the back is open. Don’t prod the shutter itself: it’s easy to bend the blades and that is bad news.
If you already reloaded the camera, you can either pray or rewind the film. It’s not likely to be a problem of development: if this happened while scanning the negative, the hair would show as white.