Hydra

So poisonous it's tracks were deadly, and having the ability to regrow two heads for every one destroyed (although one was immortal), the Hydra was a formidable monster that lived in the swamp near Lernae. To make matters worse for any would-be monster slayers, the Hydra had an ally, a giant crab.


Despite the seemingly impossible odds, Hercules and his nephew Iolaos were able to slay the monsters (and safely buried the immortal Hydra head under a rock). Although unable to kill the heroes, Hera rewarded the Hydra and Crab for their efforts by placing them in the heavens as the constellations Hydra and Cancer.



Carcinus

An ally of the Lernaean Hydra, Carcinus rushed to the she-serpent's aid when the hero Hercules attacked her, but was soon crushed. Carcinus' history is unknown -- we aren't told who he's related to (he may very well just be a normal crab), if he was acquainted with Hera like most monsters that fought Heracles, or why exactly he comes to the Hydra's aid.



Cerberus

Cerberus guarded the gates of Hades, preventing the dead from returning to the world of the living. Notably, he was the only child of Typhon and Echidna to survive an encounter with the hero Hercules.



Orthrus

Orthrus, along with the herdsman Eurytion, guarded the red cattle of the "sunset" island Erytheia. Orthrus was owned by the three-bodied giant Geryon, a close relative.



Ladon

Charged by Hera with guarding the Golden Apples of the Hesperides, Ladon was eventually defeated by Hercules.