The Quilen one. Basically in old Vale for some reason whenever a granite quilen dies, they immediately respawn at the northern gate and run to their spot in the Vale. It looks awesome. Makes you feel like those quilen hounds are really invading the zone. However if you kill a quilen, then immediately kill him at the gate again, he insta-respawns, ad infinitum. You can "catch" all quilen by the stairs where they spawn. Place a monk statue and you get an infinite horde of quilen which can't escape. They're level 90 amyway so they die to a fart. They have full loot. Can drop skyshards and for some reason can drop mogu cache keys. That's the kicker to be honest. Each key opens an insta-respawning ancient cache (they're all in one small room). It contains about 30g and a bag of panda materials. 1/60 give a skyshard though. You're basically farming every material in pandaria, maybe leathers excluded, through those zerging quilen. It's obscene.