What does the sow's head symbolize in Lord of the Flies?
The sow's head in Lord of the Flies is both a literal and figurative element in this novel. Literally, it is the head of a dead sow which Jack has his hunters prop up on a stick as a kind of sacrifice to the "beast" on the island. Jack is reasonably sure there is no particular beast to appease, but his hunters are not so sure. He directs them to leave this sacrifice in order to help them feel more at ease on the island, as if this gesture could protect them from evil. It is literally just a gory head from a sow which has been placed on a stick.
The placement of the head is fortuitously close to Simon's secret "hideaway." When he goes there after the head has has time to draw a fog of flies (though its very existence is unknown to him), Simon has one of his fainting spells and the head appears to talk to him in a vision. Figuratively, the sow's head becomes the Lord of the Flies, taunting and threatening Simon about revealing the truth--that they are the beast. Simon understands this, and the pig head tells him no one will listen to him and warns him not to tell the others what he knows or he will die.
This is exactly what happens shortly after, of course, so the Lord of the Flies acts as a foreshadowing of things to come, as well. Golding himself intended this awful figure to be indicative of Satan and the worst of our sin nature. It's an effective image, I think, if that was his intention.
As auntlori points out, the sow's head is representative of evil and is often compared to Satan.
Interestingly, Beelzebub is sometimes used as a synonym for Satan; literally translated, Beelzebub means "lord of the flies." Obviously, Golding had this in mind when he named his novel.
Many critics compare Simon's encounter with the sow's head, or lord of the flies, to the Biblical Temptation of Jesus (Matthew 4:1), in which Jesus is led into the desert and is tempted by Satan. According to the Bible, Jesus sacrifices himself to save all of humanity. Likewise, in Lord of the Flies, Simon dies trying to tell the others that there is no beast--and what they thought was the beast is really a dead parachutist. With his new knowledge that the beast lives within all of the boys, and therefore isn't something that they can hunt and kill on the island, Simon descends down the hill and, ironically, is mistaken for the beast. He is killed before he is able to give the boys information that could possibly save them.