What is it with this 3-Headed Dog, the Hound of Hades?

KERBEROS (or Cerberus) was the gigantic hound which guarded the gates of Haides. He was posted to prevent ghosts of the dead from leaving the underworld. Kerberos was described as a three-headed dog with a serpent’s tail, a mane of snakes, and a lion’s claws. Some say he had fifty heads, though this number might have included the heads of his serpentine mane.
Herakles was sent to fetch Kerberos forth from the underworld as one of his twelve labours, a task which he accomplished through the grace of Persephone.

[Nice to see a tongue-in-cheek and light-hearted approach to mythology]

from Godchecker
Another TYPHON and ECHIDNA production (like his brother ORTHRUS), CERBERUS belongs to HADES and has the disposition of a pit-bull rottweiler in a butcher’s shop.
Eyes: clear, bright and vicious.
Tongue: a healthy red and very slobbery.
Claws: highly polished. Coat: sleek, glossy and reptilian.
Tail: poisoned and barbrous.
Teeth: er, let’s not go into too much detail here.
Only HERACLES ever managed to take the Hell Hound for walkies. In fact only three other beings who weren’t dead managed to get past him at all. PSYCHE charmed him with her beauty and a doggie choc. The SIBYL-OF-CUMAE gave him a cake containing a funny substance, and ORPHEUS lulled him to sleep with his lyre.