Toughened Hide (3.5e Feat)
From D&D Wiki
Your skin is like armor.
BAB +8, Survival +11
You gain a +6 bonus to natural armor that stacks with all other natural armor bonuses.
A fighter may select this feat as one of his fighter bonus feats. A character with this feat fulfills the natural armor prerequisite for the Improved Natural Armor feat and other feats that require any sort of natural armor.