Gorillas in Stranglethorn Vale
This area is somewhat out of the way, so there will be less competition here for classic thick leather farming. There is an elite Gorilla on the island, but you can easily skip it. They might also yield the occasional Rugged Leather, though this spot is optimized for Thick Leather.
Yetis in Feralas
The Yetis in this cave are one of the very few mobs in classic wow that are humanoid and are skinnable. So they drop cloth, which makes this an amazing spot for your classic thick leather farming!