I was looking for this one. Three angles for a potential retort:

a) Frankenstein is the real monster in the book

b) The monster is Victor's son, so inherits the family name and thus is also (a) Frankenstein

c) A modern adaptation gives the reader explicit permission to use "Frankenstein" as the name for the monster: https://xkcd.com/1589/

I prefer to calm them “Frankensteins”, like:

“Look out, there’s a Frankenstein to push into that lake!”

But can never resist the Frankenstein’s monster pedantry, I find it hilarious.