WHAT IS IT?
The Demon Stingerfish (Inimicus caledonicus) and also known as Demon Stinger or the Bearded Ghoul .
It isn't one of the larger in the oceans but the fact that the long dorsal spines running down its back can inflict a painful wound means that it was always destined to be one of the more memorable.
The interesting thing about this wee beastie is that while poisonous land animals go out of their way to show that they are dangerous with bright colours, loud sounds and, if all else fails, a slick advertising campaign our fishy friend has selected a colouring that could teach the armies of the world something about the art of camouflage.
It has no known natural predators since anything that ever considered eating it was most likely very drunk at the time  but if it is disturbed it will fan its fins as a warning which is the oceanic equivalent of loading the shotgun with rocksalt.
It uses the camouflage to lie around on the ocean floor and wait for something edible to wander past  but anything that puts that much effort into hiding obviously has a past with a capital P.
WHERE DO YOU GET IT?
If you go swimming in the tropical marine waters of The Eastern Indian Ocean to the Western Pacific, generally around Northern Queensland. 
It prefers shallow waters to the cold Stygian depths, I state with confidence that it has done this on purpose because our underwater screams of pain are sweet sweet nectar to this monster
USING IT IN REAL LIFE:
Not surprisingly there doesn't seem to be much that you can do with a poisonous fish in the real world  apparently you can prepare it as a sort of poor mans Fugu  but since mankind eats pretty much everything on this planet this isn't saying much.
Possibly you could keep it as the more macho kind of pet but I recommend obtaining one through the pet shop rather than trying to go through customs with this thing down your jeans.
USING IT IN WRITING:
It would be easy to put a supernatural twist on this little fishy and turn them into an actual demon for your hero to fight.
The most important part of the fish is the venom Perhaps, when it harvested corrrectly it distills into a powerful toxin that causes an especially terrible death. An assassin would find this of interest when tasked to deliver a message to a rival as would any who investigated the murder.
Alternativly it might be that the poison, when applied in a careffully concentrated dose, is a curative and can neutralize another poison. However the use of this is almost certainly a move of the utmost desperation.
The fish might be used as an impromptu anesthetic in order to allow emergency surgery. Again, utmost and final resort.
MIGHT ONE CONQUER THE WORLD WITH IT?
is also for
Ahh, Roald Dahl, author of so many books that remain beloved of children and adults  Charlie and the Chocolate Factory, Fantastic Mr Fox, The BFG and many others were his creations and the world is a richer place for them all.
But if you only thought that he wrote for children then you, my friend, are making a grave error. Roald Dahl's other works keep the humour that you'll find in his childrens books but the macabre dial is often turned up to 11.
I heartily recommend the childrens books to anyone with young children that are ready to take the next step in their reading levels. The adult ones, not so much.
- But is a big softy really.
- And very dead afterwards
- This is a fish that doesn't go out hunting but stays very still so it can go “ Surprise!” Its those fleeting moments of terror that make its dinner all the sweeter.
- Its a sad fact that everything in the ocean wants to kill you, even the dolphins, better come to New Zealand. The worst we have are hunting kiwis and the migrating tribes of Hobbits.
- A rather loose definition but better than nothing
- The real stuff being made with Pufferfish and only by people who have had special training. Don't catch the thing and then just slap it on the grill is what I'm saying.
- Sometimes the only difference is the size of the trousers