It is very peculiar. Strange how its shape changes as it passes by.
Drone?
Definitely a UFO... It's unidentified. It's flying. It's an object. That makes it a UFO. However, I saw nothing in that video that gave me anything that made me think it was not in front of the aircraft instead of behind it. Could have been a bird, which would explain the change in shape (wings of the bird changing aspect of the bird as it moves relative to the camera position), or an insect even closer. It could also be an artifact of reflection on the lens of the camera.
I find it interesting that it is in the same angle of flight as the aircraft while the aircraft is taking off. . why as that? That's one of the reasons why I think it may be a reflection in the camera lens system. . . actually reflecting something from the plane off an interior lens back into the camera.