Role in the film
Humongous serves as a, well, "humongous" defense against city intruders. He is composed of two body halves, each half on one door of the inner gate to the Goblin City. When the doors close, the body fuses together and breaks free, attacking with a giant axe.
Humongous has a deep, reverberating voice and only seems to say "Who goes?" Interestingly, no one tries to answer, which might very well have stopped the attack, given the Labyrinth's penchant for Obvious Answers as solutions.
Humongous is revealed to be a mech suit piloted by a tiny goblin, even smaller than Hoggle, who reveals wings as he flutters down when defeated and the head of Humongous is ripped off. It is unclear whether Humongous was the one speaking or whether it was the goblin with its voice mechanically altered, as the goblin has a larger vocabulary.
In the manga sequel, Return to Labyrinth, Humongous is among the players in a sports game held to entertain Toby, though curiously smaller.