Skin

From DwarfFortressWiki

Skin is created whenever a creature without the [NOSKIN] Creature Token is butchered.

When a skin is created a "Tan a raw hide" task will be automatically generated at a tanner's shop if Auto Tan is enabled and there is a Dwarf with the tanner labor enabled. He will then take the raw hide to the workshop and turn it into leather. If left too long, a raw hide will rot and emit miasma. Its a good idea to have a refuse stockpile near your tanner's shop which accepts fresh raw hides, but not rotten raw hides. If the tanner is too busy, then refuse haulers will take away the hides that start to rot.

Make sure the tanner's shop is near the butcher's shop since every second counts.