It's a Dwarf Water Dog Necturus punctatus (four legs and gills)
Your photo of the Hellbender must have been taken
further to the W. and up in the cooler mountains.
The water dog is found a little further to the east and coastline.
Looking at the range maps it doesn't look like they overlap. Maps have been know to be wrong.

The Siren and Greater Siren do not have rear legs but have external gills.
The Amphiuma has four "legs" but no external gills.
The Water Dogs and Mud Puppys have four legs and external gills.

Can you tell me what month you found the Hellbender?

ebnagel@hotmail.com