WMG / Lisa Frankenstein

The events of Frankenstein actually happened in this universe, and furthermore...

The creature is a Frankenstein. His gravestone has the name on it, and it seems to be done for more than just the title card as it still appears as that later on. It would also explain how a corpse reanimating might even be possible in this universe, as it happened over a century ago, just under slightly different circumstances. Which one of the Frankenstein's he is might be more tricky. Elizabeth and William are out of the equation due to gender and age at death respectively, Victor froze to death in the Arctic, which doesn't align with the backstory of the creature in the movie, so the most likely option would be Ernest, who does not die in the novel.