This description is about Cerberus, the three-headed guardian of the Underworld in Greek mythology. He is a fierce and powerful creature, often depicted as a giant dog with three heads, a serpent's tail, and a mane of snakes.
From Wikipedia
In Greek mythology, Cerberus, often referred to as the hound of Hades, is a multi-headed dog that guards the gates of the underworld to prevent the dead from leaving. He was the offspring of the monsters Echidna and Typhon, and was usually described as having three heads, a serpent for a tail, and snakes protruding from his body. Cerberus is primarily known for his capture by Heracles, the last of Heracles' twelve labours.