The Jack-O-Fish is a Rare fish that was available exclusively during the Halloween Event. Since the event has ended, it can no longer be obtained through normal gameplay,

Obtainment:
The Jack-O-Fish could be obtained by Fishing during the Halloween Event. It is currently unobtainable because the event has ended.

Additional Information:
Using Abyssal Bait was the recommended way to increase your chances of catching the Jack-O-Fish while it was available during the Halloween Event.