Polyphemus

Polyphemus (Πολύφημος in Ancient Greek) is a blind Cyclops who inhabits an island in the Sea of Monsters. He stole the Golden Fleece, in order to make it easier for him to eat. This meaning that the Golden Fleece has a natural smelling scent (which gives off the same, if not extremely similar scent) of the nature god Pan (who is a son of Hermes). Pan disappeared centuries ago, until Percy, Grover, Nico, Rachel and Annabeth discovered him hidden in the Labyrinth. Satyrs would try to find Pan and since the Labyrinth blocked off any scent of Pan, the only thing giving off his scent would be the Golden Fleece. So satyrs searching for Pan would travel to Polyphemus's island, but instead would be trapped in Polyphemus' island and meet their death.

History
Polyphemus was introduced in the story in which the famed hero, Odysseus, was about to be devoured by the Cyclops. Odysseus tricked the Giant into getting drunk on honey and wine and then told Polyphemus that his name was Nobody; which would play out later. While the Cyclops was asleep with a belly full of wine, Odysseus and his men plunged a red hot poker into his single eye. Polyphemus woke with a terrible yell, screaming "Nobody has blinded me! Nobody has blinded me!" All of the other Cyclops on the island thought Polyphemus was just shouting out the obvious and paid no attention to his cries of agony. When the Cyclops opened his cave so that his sheep could go out and graze for the day, he felt each of there backs to make sure that none of the men could escape; but, Odysseus and his men were tied to the underside of the sheep and thereby escaped the Cyclops. Upon leaving the island, Odyseus revealed his true name to Polyphemus who had been told by a seer about him. Polyphemus then invoked a curse upon Odyseus in the name of his father Poseidon which just furthered Poseidon's hatred of Odysseus.

The Sea of Monsters
The only satyr who was not devoured by the glutton was Grover Underwood, who put on a wedding dress, and impersonated a female cyclops. Polyphemus then tried to marry Grover, despite the delays Grover tried to trick him with. Polyphemus was not able to tell that Grover was not a real Cyclops, because of Odysseus. Clarisse exposed Grover for being a satyr infront of Polyphemus. Polyphemus promptly decides to eat Grover and marry Clarisse insted. Polyphemus was tricked by Annabeth saying that she was "Nobody", and Polyphemus wanted his revenge (he got it by nearly killing Annabeth), and in doing so, he was almost stabbed in the eye, again. But Percy was reluctant in some ways, and decided not to. Polyphemus is also a son of Poseidon and since he seldom sees another cyclops he tried to persuade Tyson to stay with him, and learn to be a "real" Cyclops. Tyson said "No", which triggered Polyphemus to throw a tantrum and sunk the boat, in which Percy, Grover, Annabeth, and Tyson were going to escape in, he celebrated thinking he finally sunk 'nobody'. In the end, Polyphemus lost the Golden Fleece, which caused his island paradise, to become the hideous object it was before. But he still had his demonic sheep to keep company.