In my mind, Cerberus (three-headed dog, guarder of the gates to the classical underworld) will always be a Jack Russell Terrier.
Seriously though, they’re the perfect choice for the guard of Hades. They’re strong, barky, aggressive little buggers who (in my experience) don’t like letting people through gates or front doors. They like being near fire too (or at least woodburners like the one in my parent’s lounge) so the heat wouldn’t bother them too much.
Also, look at this face. Clearly the cold, dead eyes of a killer:
Pure evil. Perfect hell hound.