Considering the film's Monster Mash, it makes so much sense that the most heroic pet monster would be the one based on Frankenstein's Monster. In the original book, the monster was a tragic character who just wanted to be loved and treated like a human. It was only because people were repulsed by his appearance and showed him no love or sympathy that he actually became a monster and started killing people. With the sympathetic nature of Frankenstein's monster, combined with the loyalty of a dog, it's no wonder that Sparky is the most heroic pet after his transformation.
