Chimaerok inhabit the Isle of Dread off the coast of Feralas. They drop which are needed for  ω τ ϖ, which is itself a part of The Scepter of the Shifting Sands questline from the Gates of Ahn'Qiraj opening. This quest gives the player which (as of World of Warcraft: Wrath of the Lich King) is the only epic cooking recipe in the game.
To reach the area where the chimaerok live, one must cross from the mainland of Feralas to the Isle of Dread, which is in the southwestern corner of the zone. The player then will travel down the eastern shoreline to a path leading into the interior about halfway down the coast. It is this path that takes the character to the area where the chimaerok are flying about. There is another path that leads into the interior from the southeastern part as well.
You can get this from the Chimaerok and the Chimaerok Devourer. Likely, you can also get it from the Arcane Chimaerok. It is not a 100% drop, perhaps about 67% chance to get it from each kill. You may need to kill all the Chimaeroks 3 or 4 times to get 20 Tenderloin. It takes some time to respawn.