Once you have trained for Expert you will need to move on to another zone between the levels 25-30. In Kalimdor, Stonetalon Mountains there is Pridewing Wyverns that spawns quite fast or you may go back to Wetlands to the same place. The best place to train or farm medium leather or medium hide in World of Warcraft would be Northern Stranglethorn or The Cape of Stranglethorn. Both are filled with beast that can be skinned. The only issue with this area is the amount of hostile players, if you don’t mind PvP then these places will do you wonders.
You will be able to skin there til you reach 185 then you’ll relocate to a level 30 zone, such as The Hinterlands or Dustwallow Marsh. And you will need to stay there til you reach at least level 200 in skinning so you will be able to learn Artisan.
Northern Stranglethorn (120-145) (level 24-27)
1. Young Panther, Young Stranglethorn Tiger
3. Elder Stranglethorn Tiger
The Cape of Stranglethorn (145-160) (level 29-32)
1. Elder Shadowmaw Panther
2. Skymane Gorilla and Jaguero Stalker