It's practically a cliche at this point to point out that the monster popularly known as Frankenstein is merely the monster and the Frankenstein is the name of the scientist. But by any common naming practice, the monster, if it would have any name, would BE Frankenstein. Even slaves and dogs shared the family names of their masters, and by all but the strictest biological standard, the sentient monster must be considered Victor Frankenstein's progeny and thus entitled to the family name. IIRC, there might be some cause to assume "Adam" is the monster's given name, but as he has no other claim to any other name, whereas the scientist and eponymous character of the original novel has "Victor", it strikes me as only fair that the so-called monster be referred to as Frankenstein. Nor would it be peculiar or uniquely incorrect to refer to similar creatures as Frankensteins. If it were, we would have armies of pedantic busybodies going around saying "Actually, 'Ford" is the inventor/engineer - technically, that's a 'Ford motorcar'" Or "Technically, 'Colt' was the name of a gunsmith, you are referring to an automatic pistol."