Cerberus

Cerberus is a giant three-headed dog that appears in the episode It's About Time, who is responsible for guarding the gates of Tartarus and keeping the evil creatures within from escaping.

Appearance
Cerberus is a giant three-headed bulldog with black fur that wears spiked black collars on each of its necks.

It's About Time
In the episode, Cerberus suddenly appeared in Ponyville during Twilight's safety procedures, where it scared many of the residents with its roars and demeanor, Twilight tried to fight him but luckily Fluttershy intervened and calmed it with her kindness and affection. Twilight then used a ball to lure it away from Ponyville and back to its home in Tartarus.

Mythology
In classical mythology, the Cerberus, or Kerberos, was a giant dog-like beast with a serpent's tail who was responsible for guarding the gates of Hades and keeping spirits from leaving. Heracles (Hercules) was sent to capture Cerberus during his twelve labors.