Gigantes

The Gigantes are the children of Gaea and Tartarus. In The Lost Hero, Enceladus mentioned that each Olympian corresponded to a Gigantes.

History
The Gigantes weren't born until after the gods won the first Olympian War and took the throne from Kronos. While it was originally Gaea's idea for the gods to defeat her own children, the Titans, she was unhappy with the way the gods ruled and and gave birth to a new set of children, the Gigantes.

It was prophesied that only a god and demigod working together could defeat this new threat to Olympus, so Zeus called upon his own son Heracles to help win this war, called the Gigantomachy.

One by one, the Gigantes fell and all were killed except Aristaios, who was turned into dung beetle by Gaea so the giant might be safe from the wrath of Olympus.

The Lost Hero
After the Second Olympian War, Gaea began to wake from her slumber and began to revive her children to once again fight the gods. Enceladus was one of the first to be brought back and was the giant that was born to defeat Athena. He kidnapped Piper McLean's father, Tristan McLean, in order to force Piper to lead Jason Grace and Leo Valdez to their deaths. However, this plan failed when Piper told her friends about her father. Jason and Leo agreed to help her and Jason fought Enceladus and defeated him with the help of a lightning bolt from Zeus, his father.

Gaea also managed to capture Hera, queen of the gods to use her power to revive Porphyrion, king of the giants and the one created to destroy Zeus, king of the gods. He is awakened, but isn't at his full strength. He runs away from the fight when Leo and Piper manage to set Hera free and she shows her true form to destroy the surrounding monsters.

The Son of Neptune
In The Son of Neptune, two more of Gaea's children have been revealed to have been revived. Polybotes was leading an army to Camp Jupiter, the Roman equivalent of Camp Half-Blood in order to destroy the demigod children. He was created to destroy Neptune and shows that he has the ability to turn water into poison. He fails in his task however, as Percy manages to defeat him with the help of Terminus, the Roman god of borders.

Gaea's other child is Alcyoneus, the bane of Pluto. He was revived in part because of Hazel Levesque, a child of Pluto. Hazel was forced to help Gaea bring her child back, but eventually sacrificed her life in order to prevent his rise for a time. He can't be killed in his home territory and makes his new home in Alaska, the largest state in America. He manages to capture Thanatos, the god of death so that Gaea can revive her monsters almost instantly and bring more souls to the land of the living through the Doors of Death. He is defeated however when Arion, a mythical horse, helps Hazel and Frank Zhang pull Alcyoneus to Canada where they manage to kill him.

In The Heroes of Olympus Series

 * Porphyrion-King of the Gigantes, Bane of Zeus (Jupiter)
 * Polybotes-Bane of Poseidon (Neptune)
 * Alcyoneus-Bane of Hades (Pluto)
 * Enceladus-Bane of Athena (Minerva)

Other Gigantes

 * Ephialtes
 * Eurytos
 * Klytios
 * Mimas
 * Pallas
 * Hippolytos
 * Aigaion
 * Agrios
 * Thoon