Let us remember that Humbaba is the monster who guards the Forest of Cedars. Gilgamesh and Enkidu, Gilgamesh's erstwhile sidekick, manage to kill him in a rather titanic struggle. However, perhaps the key to any symbolic significance that Humbaba might have lies in the description that we are given of him. Enkidu famously describes him using the following language:
When he roars it is like the torrent of the storm, his breath is like fire, and his jaws are death itself. He guards the cedars so well that when the wild heifer stirs in the forest, though she is sixty leagues distant, he hears her.
If we examine the account of their battle together, we can see that Humbaba is a creature of evil. Even at the point of his death, just before Gilgamesh is going to kill him, he tries to stop Gilgamesh by promising him to serve him as a slave, and therefore seeks to manipulate him. It is only Enkidu who manages to prevent this from working. Perhaps therefore the symbolism of Humbaba lies in his evil nature: he is an aberration of nature in his physical appearance, and this indicates that he is a force of wickedness and destruction.