You can use it as a fast track guide if you do just ignore the "Your level" bit that is for people skinning as you level.You can wait until your level 50+ before trying this though, as it'll will takes you ages to complete at lower levels fast (unless of course you have a couple of level 50+ friends who don't mind grinding mobs for you to skin). I've tried to list mobs that should be yellow or orange for your skinning level. If they are green, move onto the next zone listed. I usually never have a problem skinning and have almost 300 skill in my 40's from normal questing.

Please note that even if a mob is red/orange to you, that this DOES NOT guarantee that you'll get a skill up. It's more likely, but not to be expected every time.


Skinning Level = 1 - Your Level = 5

Horde

Undead = Hounds around Brill
Orcs and Trolls = Mottled Boars and Scorpids around Sen'Jin Village
Tauren = Prowlers and Plainstriders around Bloodhoof Village

Alliance

Humans = Boars around Goldshire
Gnomes and Dwarfves = Crag Boars and Wendigo's South of Kharanos
Night Elves = Nightsabers and critters around Dolanaar


Skinning Level = 25 - Your Level = 10

Horde

Undead = Worgs and Moonrage Gluttons around Silverpine
Orcs and Trolls = Boars, Scorpids, Crocolisks and Raptors around Orgrimmar
Tauren = Prowlers and Praire Wolves around Thunderbluff

Alliance

Human = Prowlers and Bears around Eastvale Logging Camp
Gnomes and Dwarfves = Wolves around Brenwall Village.
Night Elves = Moonstalkers and Bears around Auberdine


Skinning Level = 50 - Your Level = 13

Horde

(at this level most Horde should be heading towards XR)
Plainstriders, Raptors and Prowlers around XR

Alliance

Humans = Goretusks all around Sentinel Hill.
Gnomes and Dwarfves = Bears and Crocolisks around Thelsamar
Night Elves = Moonstalkers and Bears North of Auberdine


Skinning Level = 100 - Your Level = 20

Horde

Stormsnouts and Thunderhawks around Camp Taurajo

Alliance

(at this level most Alliance should be heading for The Wetlands)
Crocolisks and Raptors around Bluegill Marsh and Whelgar's Excavation Site
Alternatives
Whelps and boars around Lakeshire for Alliance.

Skinning Level = 130 - Your Level = 24

Alliance and Horde

Bears and Mountain Lions around Hillsbrad Foothills
Alternatives
Ravagers and Wolves in Duskwood if your Alliance.
Bears, Stags and Ghostpaws in Ashenvale.

Skinning Level = 150 - Your Level = 28

Alliance and Horde

Yeti Cave in Hillsbrad Fields and Mountain Lions on the "Daggers" (these are the bits of land between Hillsbrad and Alterac Mountains, one is called Corrans Dagger, so hence the nickname)
Alternatives
Hyenas, Lions, Wyvern and Thunderhawks in Thousand Needles.
Bears, Stags and Ghostpaws in Ashenvale.

Skinning Level = 170 - Your Level = 32

Alliance and Horde

Raptors in Arathi Highlands.
Alternatives
Turtles and Basilisks in The Shimmering Flats.

Skinning Level = 180 - Your Level = 34

Alliance and Horde

Raptors, Panthers and Tigers around Nessingwarys Camp in Stranglethorn Vale
(you could actually get your skinning level all the way to 300 in and around STV as the mobs range from level 34 to level 50)
Alternatives
Raptors and Crocs in Dustwallow Marsh.
Thunder Lizards, Kodos, Basilisks, Scorpids and Hyenas in Desolace.
Whelps in Swamp of Sorrows.

Skinning Level = 220 - Your Level = 38

Alliance and Horde

Raptors and Gorillas around the Gurabashi Arena
Alternatives
Panthers, Jaguars and Crocs in Swamp of Sorrows.
Dragonkin, Coyotes and Ridge Stalkers/Huntresses in Badlands.
Wolves around Feathermoon Stronghold in Feralas if your Alliance.

Skinning Level = 250 - Your Level = 45

Alliance and Horde

Snickerfangs and Boars in North Blasted Lands
Alternatives
Silvermanes and Gryphons in The Hinterlands.
Stags and Hippogryphs in Azshara.
Hippogryphs, Apes, Bears, Wolves, Screechers and Yetis in Feralas.

Skinning Level = 275 - Your Level = 50

Alliance and Horde

Jagueros and Gorillas on Jaguero Island (Large Island to the East of Booty Bay where Princess Poobah is being held). Skin till 300.
Alternatives
Yetis, Chimeras and Bears in Winterspring.
Bears in Western Plaguelands.
Plaguehounds and Plaguebats in Eastern Plaguelands.
Turtles by Raventusk Village in The Hinterlands.
Turtles and Chimeras in Azshara.
Dragonkin, Worgs and Scorpids in Burning Steppes.

Thought I'd add the mob levels that you can expect to get each specific leather from.

Ruined Leather Scraps: 1-16
Light Leather: 1-27
Light Hide: 10-27
Medium Leather: 15-36
Medium Hide: 15-36
Heavy Leather: 25-46
Heavy Hide: 25-46
Thick Leather: 35-63
Thick Hide: 40-59
Rugged Leather: 43-63
Rugged Hide: 47-63