Skinning is a main profession as many of you know, it is normally coupled with Leather Working in order to make Leather and Mail armor. If you do not have Leather Working as your other profession then as you get the leathers your best choice is to sell them on the auction house. On the majority of realms you can get a good price for leathers so you can make quite a bit of gold grinding them, you could also make rare Leather Worked items with the leather and sell them on the auction house. Now then, this guide will give you some areas to farm leather quickly.
Scion of Quetz'lun - NPC - World of Warcraft (74,59) Zul'Drak - Scion of Quetz'lun
These are the only monsters in the entire game to give more than
1 Borean Leather, it is recommend that you are level 80 and have 400+ skinning to make this place worthwhile because the mobs are elites and have a bit of health. They also have a good Artic Fur drop rate which sell very well. On top of this, they drop good trash which can get you some more gold.
Primordial Drake - NPC - World of Warcraft (45,26) Sholazar Basin - Primordial Drakes
These Primordial Drakes do not have much health and there are plenty of them, they drop
a lot of trash which gives you some gold but when skinned they have a decent chance, around 1/4 to drop Icy Dragonscales which are used to create frost resistance gear and also for a couple of good Hunter items. Also a good place to farm chilled meat. If you crack open the eggs there a hatchling will appear and you can kill and skin those too.
Icetip Crawler - NPC - World of Warcraft (48,71) Storm Peaks - Icetip Crawler
These spiders are at the back of the cave which is slightly north of the k3. These spiders are skinnable and have an extremely fast respawn rate, drop a lot of trash. There is also a quest for them in the cave which can get you 6g. This isn't a great spot but it isn't bad. Not many people come here so it can be a decent site if the others are being camped.
Cavedweller Worg - NPC - World of Warcraft (48,52) Storm Peaks
These worgs used to be the best spot to farm leather and artic fur, however blizz severely nerfed their respawn rate. This isn't too big of a problem if you work your way to the back of the cave and then to the front again. The worms here are skinnable too but its best to stick to the worgs.
Ice Steppe Rhino - NPC - World of Warcraft (48,61) Storm Peaks
These rhinos walk in packs just south of the cave with the worgs and worms in. It is useful if there is someone already in the cave killing the worgs. Also a very good place for rhino meat. Easiest way to take them down quickly is to AOE them.
These are just some spots that you can use to farm leather and if you have anymore please post them. I'm sorry there are no pictures but my gametime has expired.