Cerberos

Kerberos (in Greek, Russie, in Latin, Cerberus) is a three-headed dog that guards the entrance to the underworld. By other accounts, he guarded the underworld above all from the entry of the living.

He is portrayed with three heads, a dragon's tail, with snakes on his heads instead of fur. He was later credited with a hundred heads, poisonous breath and a multitude of snakeheads on his necks. But the number of Kerber's heads is unclear. Hesiodos only says he has countless. As a triple-headed, it is most commonly cited only in Roman mythology. Kerberos saw the light of day only once, when he was led out of the underworld by Heracles in the pursuit of the twelfth task. Mycenaean King Eurysthea asked Heracles to bring him Kerber. With the help of the gods, Heracles had reached the underworld, had been kindly received by Hades itself. He allowed him to catch Kerber on the condition that he could do it with his bare hands. Kerberos fought back ferociously, but after Heracles' strong grip he promised obedience and followed him to Mycenae. When King Eurystheus saw the terrible guardian of the underworld, Kerber, he fell to his knees in front of Heracles and begged to take the monster back.

Only Orpheus bewitched Kerber with his singing, and Aeneas and some other selectors allowed by the gods to enter the underworld soothed him with honey pie.

Kerberos was the model for the three-headed dog, Hazel J. K. Rowling in Harry Potter.