Hydra

The Hydra (ヒドラ, Hidora) is a mythological creature with seven cavezas that Hercules had to fight, the feature is that every time a head was cut off two took its place. Appears in Episode G and Gigantomachia.

Episode G
When the body of Kronos was released, Aiolia and Shura had to face the six Titanids, drawing his wounds Themis tried to crush them with their technique Brabeus Talanton but Taurus Aldebaran came and saved them to finish them snakes Rea called rock, but they were destroyed, so called Hydra, Capricorn Shura used his Excalibur to cut some heads but when the Saints were doing their blood splashed acid appeared as new heads, then used the Taurus Aldebaran Great Horn to attack the main head, which allegedly was immortal, and completely destroyed killing Hydra, incidentally ended Themis technique.

Gigantomachia
It is one of the Gigas Shinshiki (Group found the Echidna and her children). Although appointed by Echidna in volume 2 of the novel Gigantomaquia, never made an appearance.