Tiny Red Carp
From Townlong Steppes. The red scales on these fish are believed to be a warning to predators of the Townlong Steppes and the Dread Wastes that their flesh is toxic. Dedi... Aquatic type. Patch 5.3: , , , and can now be caged.
The 
Tiny Red Carp is a fishing profession pet obtained by casting into open water in the Townlong Steppes, considered by Blizzard to be the rarest battle pet in the game.
Guide awaiting review
Travel to Townlong Steppes
Head to the pond next to Niuzao Temple in Townlong Steppes — this is the confirmed location where the Tiny Red Carp can be caught, near the pet daily quest NPC.
Fish Open Water Only
Cast into open water, not fishing pools — a player confirmed catching their Tiny Red Carp from open water at 34, 52 in the Fields of Niuzao. Spinefish Schools have been suggested as a possible source but have not been confirmed by any player.
Maximize Your Fishing Skill
Raise your fishing skill to at least 725 using gear such as the 
Weather-Beaten Fishing Hat, 
Nat Pagle's Extreme Anglin' Boots, and fishing lures before casting, as catch rates are extremely low and players have reported anywhere from 300 to 5,000 casts before landing this pet.
