The Collaborative International Dictionary of English v.0.48:

Hellbender \Hell"bend`er\, n. (Zool.)
   A large North American aquatic salamander (Protonopsis
   horrida or Menopoma Alleghaniensis). It is very voracious
   and very tenacious of life. Also called alligator, and
   water dog.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hellbender
    n 1: large salamander of North American rivers and streams [syn:
         hellbender, mud puppy, Cryptobranchus alleganiensis]