In fetters Hercules fierce Cerberus tied,
And took him trembling from grim Pluto's side,
From realms of darkness drag'd away to light,
The yelling monster sicken'd at the sight,
And from his jaws the foam which fell to earth,
Unto the poisonous Aconite gave birth.
In Greek mythology, Cerberus or Kerberos (Gree Kerberos, "demon of the pit") was the hound of Hades, a monstrous three-headed dog (sometimes said to have 50 or 100 heads) called a hellhound. Cerberus guarded the gate to Hades and ensured that spirits of the dead could enter, but none could exit (additionally, no living person was to come into Hades).
Heracles' final labour was to capture Cerberus, which he did by wrestling it into submission.