The gorgons: We saw the gorgon Medusa in The Lightning Thief. The infamous Medusa had two sisters named Euryale and Stheno. All three of them were gorgons, monster-ladies with snakes for hair and usually part-serpent bodies. Medusa had an extra-special curse that turned people to stone if they looked at her. Euryale and Stheno are just terrifying and will probably eat you. The Son of Neptune opens with the two of them chasing Percy, partly for fun and partly to avenge their sister. 4/5 Monstrous Rating for their Bargain Mart disguise.
Karpoi: The karpoi (or carpi) are spirits of the grain/fruit. They work primarily for Demeter/Ceres (goddess of agriculture, etc) and Gaea (Titan of the Earth). They’re depicted as infants, kinda like little angry cupids that grow plants. Gaea recruits them to her side in The Son of Neptune by promising them all the land they need to grow ALL the grains! They’re really annoying, angry, and tiny, but I like watching them argue about which grain is best. 3/5 Monstrous Rating.
Gegenes the Earthborn: These are six-armed giants from Greek mythology, not Roman, which is confusing because why are they in this Percy Jackson series? Whatever. The Argonauts (Jason and Co. who went after the Golden Fleece) had to fight them. They’re part of Gaea’s army but so far haven’t done anything else. 3/5 Monstrous Rating.
Cyclopes: We saw these guys in Sea of Monsters. Cyclopes are one-eyed giants, and while they seem to be herdsmen when in their own country, Zeus and Hephaestus use them as workmen at their forges. They supposedly forged Zeus’ famous thunderbolts, but they also seem to enjoy eating humans when the opportunity arises. I like how Riordan gives us the good and bad extremes of Cyclopes, since the myths seem undecided on them, but the cyclopes in TLH are pretty monstrous. The scene where they try to cook and eat Jason and his friends reminds me a lot of Bilbo and the trolls in The Hobbit. 3/5 Monstrous Rating for being pretty terrifying but also pretty dumb and over done.
Centaurs: We saw centaurs in the last series of Percy Jackson books, but in this series they’re the Roman version of Centaurs. Roman Centaurs have a greater tendency to murder all your men and rape all your women and steal all of your stuff, as opposed to the hero-trainer Chiron or other Greek centaurs that can be really drunk and out of control but not usually as violent. Percy Jackson is PRETTY UPSET to find that the centaurs are allying themselves with Gaea in The Son of Neptune. 3/5 Monstrous Rating.
Polybotes: Polybotes is a giant and happens to be a (im)mortal enemy of Poseidon (Percy’s dad). He’s a little pissed because during the war between the gods and the giants, Poseidon dropped an island on top of him. Polybotes has it out for Poseidon and all of his descendants. He’s a Big Bad in The Son of Neptune, leading an army to attack Camp Jupiter (home of the Roman demigods including Frank and Hazel). 4/5 Monstrous Rating.
Alcyoneus: Yet another giant, and a son of Gaea, Alcyoneus was said to be invincible in his home territory. He fought Hercules, who only managed to kill him by shooting him and then dragging him out of his territory to die. Cold, Hercules. The heroes in The Son of Neptune employ a similar tactic against him. That being said, he is pretty terrifying, and Gaea goes to a lot of effort to bring him back from the dead. 4/5 Monstrous Rating.
Basilisks: Basilisks are not terrifying giant dragons, like you may have been led to believe, but relatively small snakes. They’re super poisonous, able to wither plants with only their breath, and obviously their venom is super lethal to anyone. In The Son of Neptune, Frank has to fight three of them while they’re hiding in tall grass. 4/5 Monstrous Rating because deadliness>size.
Arion: Arion is the son of Neptune/Poseidon and Ceres/Demeter. At some point Poseidon decided he was in love with Demeter, and even though she ran away and turned herself into a horse to escape him, he decided to turn into a horse himself to get what he wanted because he’s terrible (gods are sometimes cool and sometimes The Worst). Their son Arion is an immortal horse and had famous riders including Hercules. In The Son of Neptune, he decides Hazel is the best (because she is) and allows her to ride him. He’s super fast and super powerful and will bite your hand off if he doesn’t like you. 5/5 Monstrous Rating.
Harpies: Harpies are wind-spirits, similar to the venti that we saw in The Lost Hero, but these ones take the form of bird-human hybrids (usually they look like birds except for their human heads. Yikes.). They like to eat other people’s food. I usually only see harpies portrayed as monsters so it was fun to have a sympathetic supporting harpy character, Aella, in The Son of Neptune. She has a photographic memory and has read a lot of books, so she is endlessly helpful. 4/5 Monstrous Rating.
Laestrygonians: You can experience these guys in all their violent glory in The Odyssey. Odysseus’ crew reaches a very promising-looking island, until they’re chased off by giants who throw rocks at them. In The Son of Neptune, Gaea tells them that if they eat Frank they will receive his superpowers, so they encircle Frank’s family mansion and lob it with fireballs. If you didn’t notice, there are SO MANY species of mythic giants and almost all of them want to eat you. 3/5 Monstrous Rating.
Spartus/skeleton warrior: Sometimes known as the Sparti/Spartoi, the hero Jason had to face these scary dudes during the Argonauts’ quest for the Golden Fleece. When the Argonauts reach the land of King Aetes and ask him for the Golden Fleece, the king can’t turn them down outright because they’ve become his guests. So he tells Jason that he has to perform a task for him: yoke some fire-breathing bronze oxen, sow a field with dragon-teeth, and kill the crop of armed men that spring up. Jason is a little taken aback by this very specific and lethal request, but he’s the one who signed up for the quest, after all. In The Son of Neptune, the demigod Frank gets a spear on loan from the god Mars which gives Frank the power to summon a spartus three times to fight his enemies. It was cool to bring these guys back to use on the good guy’s side, although Frank probably should trust his spartus too much. 5/5 Monstrous Rating.
Gryphons: Gryphons or griffins usually have the body of a lion and the wings and head of an eagle. There are some variations, and in The Son of Neptune the gryphons that attack our heroes have the body of black panthers rather than lions. I don’t really know why except to be scarier, maybe? I can’t find any evidence of actual myth griffins with panther bodies, but it seems reasonable. Apollo likes to ride griffins probably because, let’s face it, they are very cool-looking. 4/5 Monstrous Rating.
Hyperborean giants: In mythology, the land of the Hyperboreans was far to the north and inaccessible to normal humans, but it was apparently a super great, heavenly place that was always springtime. In The Son of Neptune, Percy and his friends come across a few of them in Alaska: they’re huuuuuuge, blue, and not very smart, but basically harmless like giant blue frost-breathing cows or something. 3/5 Monstrous Rating because I want to be friends with them.
Phineas/Phineus: Phineas was a little too good at predicting the future, so Zeus cursed him to be haunted by harpies. These harpies would steal his food no matter what, so that Phineas could never eat. He was rescued from the harpies by the Argonauts. In The Son of Neptune, Gaea has brought him back to life and he is working for her with his prophecies.
Otrera: Otrera was the first queen of the Amazons. Depending on the story, she is either Ares/Mars’ wife or daughter (yikes how do those get confused?). She is killed by the hero Bellerophon (he notably killed the Chimera, and befriended Pegasus). In The Son of Neptune, Gaea brings her back to life to work for her, and Otrera is attempting to take over the queenship of the Amazons again.
Bulfinch, Thomas. Bulfinch’s Greek and Roman Mythology: The Age of Fable. Dover Thrift, 2000. Print.
Hamilton, Edith. Mythology. New American Library, 1969. Print.
Riordan, Rick. The Son of Neptune. Disney Hyperion, 2011. Print.
See also the links above for more sources!