This is one of three quests you can get from fishing quest starters in Pandaria.
Usually I catch the [item=86545] on the coast between Krasarang Wilds and Jade Forest.
The other two rare fishes are:
[item=86544] and [item=86542]. They are unique and you can complete the quests once per day to increase your reputation with Nat Pagle and The Anglers. By now this is the only way to make Nat Pagle your Buddy and get his special faction rewards. After giving away your rare catch of course you can try to get a new one for next days turn in.
You'll also need these 3 rare fishes for Pandarian Angler.
Build 15929: At the moment only Flying Tiger Gourami is working. When trying the other two you only loose your quest starters :-(