I've never managed to convince anybody of this, but... while I think you can't really say that the monster's name was Frankenstein, you would be well justified in calling the monster "a Frankenstein," after its creator, in the same way you'd refer to a Garand (the rifle) after its creator (John Garand). As in, "Frankenstein is a novel about Dr. Frankenstein, who was the first person in fiction to create a freaky Frankenstein, whom he later rejected."