This special fish is used in the quest to increase your Fishing skill cap to 300: Nat Pagle, Angler Extreme. It is one of the few Soulbound fish and is also Unique.
This fish is found in the Verdantis River in Feralas. The river is in central Feralas, west of Camp Mojache, where the mountains dip south. Make certain the label on your mini-map says "Verdantis River" or you are in the wrong place.
The Verdantis River is difficult fishing for a 225 skill character and you will receive many, many failures. Its probably advisable to pick up some 
[Bright Baubles] or even an 
[Aquadynamic Fish Attractor] to speed up the search.