Guard of Hades' Underworld,
The famous multi-headed dog of Greek mythology, Cerberus (or Kerberos in the original Greek) is the pet of Hades and fearsome fanged guardian of the Underworld who prevents the dead who've crossed the River Styx from ever leaving. Descriptions of the monster vary depending on the source: sometimes cited as having two heads and sometimes as many as 50, though three is the most common, magic number. Cerberus is also said to have a tail of snakes and aside from being an iconic ancient Greek beastie in art through the ages, the big dog has the honour of being the last of Heracles' twelve labours. On the condition that he can overcome the colossal canine without weapons (which he does) Hades allows the hero to borrow Cerberus to show to taskmaster King Eurystheus who subsequently hides in a jar and accepts that Heracles has achieved the obligation. Dante's Inferno has the hound watching over the glutinous in the Third Circle of Hell and I'd say -
because three heads are better than one - Cerberus stands proud as the greatest guard dog ever.