Phil Foglio on "The Galactic Ninja Template" From Buck Godot: Zap Gun for Hire. Phil is also the author of the very popular Girl Genius comic.
But there are things they all have in common:
They are usually very dangerous (at least to other members of their race. The most dangerous P'Thhard in the universe will still get stepped on by anything over one meter tall). They are very adept at concealment (It is now known than Ooonge ninja were responsible for the destruction of the first seventy five landing parties on their planet, who never realized that they were anything more than just another mountain range.) They possess abilities that are normally not associated with their races (Stlangworz ninja have been reported walking and chewing hebz roots at the same time) and they are utterly convinced that theirs is the only true way and that all other ninja are incompetent posers.
Because of this they all hate each other. This, galactic ecologists assure us, is what keeps their numbers down to acceptable levels.