When they left the pig's head there after they killed it in Chapter 8, what did it symbolize?
Simon is terrified of the pig's head on the stick. He sees it as a symbol of evil- the savagery of the boys when killing the pig unleashes the evilness among them. The flies that swarm are what gives it the term "Lord of the Flies". It symbolizes dark forces, like the devil.
When the boys leave the clearing where they killed the sow, they decide to leave an offering to the Beast. They remove the pig's head and impale it on a spear, which they then place in the ground. But, the clearing where the boys killed the sow is also the clearing where Simon frequently goes to be alone. In fact, when the sow is killed, Simon is hiding in the leaves, watching. When the hunters leave, Simon emerges and stares at the pig's head. It is as if the head has put him into a kind of trance. The flies that have gathered swarm around the head, attracted to the blood. Simon beings speaking aloud and imagining that the head is talking back to him. Golding refers to the head as the Lord of the Flies.
The author's choice to call the head "Lord of the Flies" is important, both literally and symbolically. Literally, the name is an accurate way of describing what Simon sees: a disembodied head with flies buzzing around it, as if the head were their master. But there is a symbolic meaning to this name, too. "Lord of the Flies" is actually a literal translation of the Greek word, "Beelzebub." "Beelzebub" is a name for the devil, the figure in the Judeo-Christian tradition that is considered to be the embodiment of pure evil. By calling the pig's head the "Lord of the Flies," Golding is making an association between it and evil.