Source : Webster's Revised Unabridged Dictionary (1913)

Hellhound \Hell"hound`\, n. [AS. hellehund.]
   A dog of hell; an agent of hell.

         A hellhound, that doth hunt us all to death. --Shak.

Source : WordNet®

hellhound
     n 1: a very evil man
     2: (Greek mythology) 3-headed dog guarding the entrance to
        Hades; son of Typhon [syn: {Cerberus}]