Gorgon

The Gorgons were terrifying monsters. Contrary to popular belief, only Medusa could turn people into stone. They were three monstrous sea spirits whose names were Stheno, Euryale, and the only mortal among the three, Medusa. Medusa was killed by Perseus by cutting off her head. Her head was later used by Perseus to kill the sea monster Cetus. Zeus created his shield Aegis on oracular advise so he could win the war against the Titans. He defeated the gorgon Aex (or Aix), who was either the daughter of Helios (with a head as terrifying as her body was beautiful) or alternatively the son of Typhon and Echidna. He used the skin of a goat and fixed upon it the head of the gorgon Aex. It was said Zeus gave Aegis to his daughter Athena once the war was over.

Becoming Gorgons
Medusa and her two sisters, Stheno and Euryale, were turned into the three Gorgons by Athena. The transformation was punishment; the human Medusa had been caught consorting with Poseidon in her temple. As Athena could not punish Poseidon, she punished Medusa. The result was a monstrous demon with slithering serpents as hair and a gaze that could turn a man to stone.

Medusa's sisters were turned into Gorgons as well for the act of assisting their sister into the goddess's sacred temple and because they were kind enough to share their sisters fate.

It is said that Medusa created statues because her sisters faded and she was left lonely. In some myths, the Gorgons were beautiful maidens with brass wings like angels.

Abilities

 * Living, venomous snakes for hair
 * Blood from their right side can cure anything, but the blood from their left side is deadly
 * Gazes so terrible, it can turn anyone affected to stone (though, only Medusa has this ability)
 * The ability to persuade people to open their eyes with a soothing voice

The Gorgons are not listed to have any other power, but in some myths they are said to have unbreakable bronze and silver wings, teeth of gold, and tails that can wrap an elephant.

The Lightning Thief
Medusa tries to turn Percy, Annabeth, and Grover into stone upon stumbling across her lair. Grover is the first to figure out that Medusa, not "Aunty Em", is their hostess. He sees a statue that looks unusually alike his uncle, and recalls that his Uncle Ferdinand was turned to stone by the monster Medusa.

Medusa holds a grudge against Annabeth because she is the daughter of Athena, the one responsible for Medusa's misery. However, Medusa told them that her sisters had faded and now she was alone.

Eventually her head was chopped by Percy Jackson as a reference to the original Perseus, and used by Sally Jackson to turn Gabe into stone.

The Lost Hero
When Chiron is explaining how the use of technology can be dangerous for demigods, he says that a demigod once tried to Google the Gorgons, causing them to appear. It is not mentioned if one or all of them appeared.

The Son of Neptune
Upon leaving the Wolf House, Percy is being chased by Medusa's two sisters, Euryale and Stheno (who Percy originally thought was named Beano), because he killed Medusa years ago (even though he can't remember it). The two gorgons keep reforming after Percy has killed them. Stheno ignorantly tells Percy about gorgon blood, with that taken from the left being deadly while from the right could heal. They also fill him in on some of the stuff readers learned in the previous book, such as Gaea, their patron opening the Doors of Death from Tartarus, and they also tell him he has the Achilles Curse, and he is the son of a sea god, which he apparently didn't know. He drowns them both in the Tiber after they try to kill Frank, but reform soon afterwards.

Medusa
Medusa is the only gorgon that has appeared so far in the series, her head was severed by Percy Jackson in the Lightning Thief, with the help of Grover and Annabeth.

It was said that Medusa was the youngest Gorgon among the three, as well as the most beautiful, which is the reason Poseidon noticed her. She is also the most famous. Medusa is also the only one with the power to turn her victims into stone, being the envy of Euryale.

Euryale
The immortal sister of Medusa. She has been known for bellowing cries. She has faded, along with the third sister.

It was said that Euryale was the middle among the her sisters. In The Son of Neptune, it is revealed that like Medea, Midas and Lycaon, Stethno, and she are released from the Doors of Death and could reform within two hours. She is the smarter of the two. She isn't the oldest.

Stheno
The immortal sister of Medusa. She has been most remembered to be the most vicious of the Gorgon sisters. She has faded with the second sister. Stheno was said to be the eldest of her sisters.

Euryale and she are released from the doors of death. She is more naive as she blurts out gorgon facts to Percy and seems to have acquired a love for her cover-up job at the Bargain Mart.

Aegis
Aegis is a bronze shield that Athena gave to Zeus that has an imprinting of Medusa, one of the Gorgons, on it. Thalia Grace has a shield modeled after it.

Trivia

 * All three of the Gorgons appear in the Percy Jackson and the Olympians video game, but Stethno and Euryale are stuck in the Underworld.