My pirate on Test was fortunate enough to have been able to catch this beasty. I'll try to go as deep in to my memory as possible:
I was on a fishing boat, parked directly off the "town" area of Padres Del Fuego. I casted by Legendary Lure out as far as possible, and repeated it some... oh, it has to be close to a dozen, but probably less. After a while, (and a battle with Speedy Lou) Dragon Fish came along, and I added him to my collection. Yay
I've heard rumors that Dragon Fish only comes out at twilight (in the game, of course.) Whether this is 100% true or not, I'm not sure.
Also, as a side note, if you do run in to Dragon Fish:
be careful, and fight hard! Dragon Fish puts all the other Legendary Fish fights to shame. If you need any tips on how to fight the beasty to the best of your ability, just ask
Happy pirati- err, happy fishing!