Ork leaders commonly display their prowess by hunting down and killing dangerous beasts. The creatures they fight against the most are those with big horns or tusks which can then be displayed on the Nob's helmet for all the Boyz to see. If such a creature has a disappointing lack of horns or tusks the Ork leader will sport a huge metal jaw instead to show that he has got the biggest bite around! If a mob is led by a Warboss or Nob with either Big Horns or an Iron Gob they add +1 to their Leadership value. Note that the two may not be combined to get a +2 bonus.
Big Horns/Iron Gob
Source: Codex: Orks, p. 34