Natural Armor (3.5e Feat)
From D&D Wiki
through a birth defect your character gains a natural armor that hardens as time goes on
Prerequisite: level 1 (must be taken at character creation), a positive constitution modifer, no previous natural armor, non-human.
Benefit: gain a natural armor bonus of +1 at first level that increases every 4 levels thereafter
Normal: specific races get a natural armor bonus
Special: gain natural armor