Cerberus was a three headed dog who guarded the Underworld allowing souls in but not back out again. Aside from having three heads Cerberus also had the tail of a serpent, a mane made of snakes and lion’s claws.
He was also the final labour given to Heracles by King Eurystheus. Heracles had to capture the dog without the use of any weapons. After eventually overpowering Cerberus, Heracles brought him to Eurystheus thus completing his twelve labours.