Guarding the entrance to Thebes, the Sphinx asked all who passed by a riddle, and devoured those that couldn't answer correctly.
Sphinx
The guardian of the Golden Fleece at Colchis, the Colchian Dragon kept constant watch, never resting for even a moment (although it could be forced to with music or magic).
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.
This monstrous pig was owned by an old hag named Phaea, and was sometimes herself called by that name. The pair wreaked havoc in the land of Crommyon, purportedly one of the entrances to the Underworld, until they were slain by Theseus. Incidentally, Theseus killed several other villains inhabiting other entrances to the Underworld, all while on his way to Athens to claim his birthright.
When the king of Calydon neglected to include Artemis in his annual sacrifice to the gods, she sent a gargantuan boar to ravage the land. The king called upon all of Greece's heroes for help --including the king's own son Meleager, and a huntress named Atalanta-- and after a difficult battle in which several heroes fell, the pig was eventually slain. Its hide was to go to Meleager, as he landed the killing blow, but he offered it to Atalanta, as she struck the first hit (and because he was in love with her). Some of the surviving heroes objected as Atalanta was a woman and soon another battle ensued, in which Meleager killed his own kin. His mother was distraught at the news and burned a cursed brand (wood used in a hearth) that was fated to end Meleager's life when engulfed in flame, and so he dropped dead on the spot. In a roundabout way, Artemis still got her revenge as the king lost his son.
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.
