Unggoy Majors lead their lesser counterparts into battle under the command of a higher ranked Brute or Elite. Although these higher ranking grunts are superior to grunt minors in strength, intelligence, leadership and morale, they will sometimes still panic and flee from enemy forces when overwhelmed.
I always wanted to make a grunt, because they just rock in the game, that's a fact. :-P It was pretty hard though, but I think it turned out well enough. (credit to Phong Chang for the head design) Although it is never seen in the game, I couldn't resist taking a shot of this little guy with a Fuel Rod Gun.