Today, many people refer to the ancient Greek diety Kerberos by his Roman name, which is Cerberus, so we will too. Whether you call him Kerberos or Cerberus, he was the three-headed dog who guarded the gates to the underworld, home of the powerful god, Hades.
Hades loved that fierce, bad tempered dog. Sometimes, just for fun, Hades and his beloved dog Cerberus would climb aboard Hades' golden chariot and tear across the underworld, scattering the souls of the dead, who had to dive left and right to get out of their way.
But mostly, Cerberus guarded the gates, making sure no one entered or left without permission. Orpheus managed to sneak into the underworld by lulling Cerebus to sleep with his beautiful music. But that only happened once. Cerberus was a very good watchdog.