Thick Leather

Thick Leather can be from skinning level 40-50 mobs. Reliable sources include:
 * Level 45+: Nyxondra's Broodlings in Badlands, with a 77% drop rate. They are only a few feet apart from each other and they will respawn before you clear the area.
 * Level 47+: Ravasaurs in Un'Goro Crater. They drop the same amount of Thick Leather as the whelps in Badlands, but they are far from each other and it's a bit harder to kill them.

Blacksmithing

 * Big Black Mace
 * Blight (polearm)
 * Blue Glittering Axe
 * Ebon Shiv
 * Heavy Mithril Boots
 * Mithril Scale Shoulders
 * Ornate Mithril Boots
 * Ornate Mithril Shoulder
 * Phantom Blade
 * Runed Mithril Hammer
 * The Shatterer
 * Truesilver Champion
 * Wicked Mithril Blade

Engineering

 * Blue Rocket Cluster
 * Catseye Ultra Goggles
 * Deadly Scope
 * Green Lens
 * Green Rocket Cluster
 * Red Rocket Cluster
 * Rose Colored Goggles
 * Spellpower Goggles Xtreme

Leatherworking

 * Big Voodoo Cloak
 * Big Voodoo Mask
 * Big Voodoo Pants
 * Big Voodoo Robe
 * Dragonscale Breastplate
 * Dragonscale Gauntlets
 * Feathered Breastplate
 * Gauntlets of the Sea
 * Helm of Fire
 * Nightscape Boots
 * Nightscape Headband
 * Nightscape Pants
 * Nightscape Shoulders
 * Nightscape Tunic
 * Rugged Leather
 * Shadowskin Gloves
 * Thick Armor Kit
 * Tough Scorpid Boots
 * Tough Scorpid Bracers
 * Tough Scorpid Breastplate
 * Tough Scorpid Gloves
 * Tough Scorpid Helm
 * Tough Scorpid Leggings
 * Tough Scorpid Shoulders
 * Turtle Scale Bracers
 * Turtle Scale Breastplate
 * Turtle Scale Gloves
 * Turtle Scale Helm
 * Turtle Scale Leggings
 * Wild Leather Boots
 * Wild Leather Cloak
 * Wild Leather Helmet
 * Wild Leather Leggings
 * Wild Leather Shoulders
 * Wild Leather Vest
 * Wolfshead Helm

Tailoring

 * Black Mageweave Boots
 * Crimson Silk Gloves
 * Dress Shoes
 * Shadoweave Boots

As a quest objective

 * Wild Leather Armor (Alliance)
 * Wild Leather Armor (Horde)
 * The Horde Needs Thick Leather!
 * The Horde Needs More Thick Leather!
 * The Alliance Needs Thick Leather!
 * The Alliance Needs More Thick Leather!