The three-headed hound is from Greek Mythology. It is called Cerberus and is sometimes referred to as The Hound of Hades. It would prevent the dead from leaving the underworld.
This episode reveals how Snow White acquired her archery skills.
In the original myths, Hercules persuades Hades to let him take Cerberus to King Eurystheus, who was giving Hercules his labors. The beast terrified the king then returned to Hades. Many years later Herc was poisoned with hydra blood through the treachery of the centaur Nessus.
Kacey Rohl (Megera) appears here as does a three-headed hell hound. She also appeared in the episode of Supernatural The Born-Again Identity (2012), which is popular for using hell hounds on the show.