Location:
Onyxia's Lair 10man
Requirements:
Skinning
Minor selfheals
How to:
Inside Onyxia's Lair there is 4 Onyxian Warders, these are easily solo-able and gives 8-20+ Rugged leather when you skin them. However, the exploit is that whenever you drag onyxia out of her lair. Two onyxian warders spawn in the entrance path to join the fight later. Before MoP came out, it was not possible to farm these Onyxian warders easily nor could you drag onyxia out without getting killed as she would enrage and flame up the entrance path. Come MoP, most classes can now quickly drag onyxia out, make her despawn, kill the two Onyxian warders, skin them and repeat.
How can you make so much gold?
Most likely because Rugged leather is one of the hardest mats to farm to level Leatherworking. The demand has usually been high enough to keep Rugged leather at 150-200g/stack (on my server). During my 20min farm I ended up with 330 Rugged leather which would mean I get 9k gold/hour with this method. Most realms it is 100g-150g/stack and this would mean 6750g/hour (at 150g/stack)
EDIT::
VIDEO TUTORIAL