Reddit user "HanA**holeSolo" first shared the GIF last Wednesday of Trump pummeling a wrestler with CNN's logo imposed on his face. CNN could find no earlier instance of the GIF. The GIF was later edited into a video with sound and tweeted by the President on Sunday.
CNN found this private citizen and threatened to expose him or her is they didn't apologize and take down the tweet. The story here > How CNN found the Reddit user behind the Trump wrestling GIF