What is “The Lord of the Flies”? Who names it?

It is Belzebub, the latin name for Satan. Golding names it.

Simon sits alone in the clearing, staring with rapt attention at the impaled pig’s head, which is now swarming with flies. The sight mesmerizes him, and it even seems as if the head comes to life. The head speaks to Simon in the voice of the “Lord of the Flies,” ominously declaring that Simon will never be able to escape him, for he lies within all human beings. He also promises to have some “fun” with Simon. Terrified and troubled by the apparition, Simon collapses in a faint.

Simon's delusions name the pig's head.

Sorry, the spelling is "Beelzebub". Here is a more detailed quote of my answer,

In this way, the Lord of the Flies becomes both a physical manifestation of the beast, a symbol of the power of evil, and a kind of Satan figure who evokes the beast within each human being. Looking at the novel in the context of biblical parallels, the Lord of the Flies recalls the devil, just as Simon recalls Jesus. In fact, the name “Lord of the Flies” is a literal translation of the name of the biblical name Beelzebub, a powerful demon in hell sometimes thought to be the devil himself.