Doctor Frankenstein was sort of a father to the Monster
So it's reasonable to assume that the Monster would take the father's name. Thus he is Monster Frankenstein, son of Dr. Frankenstein.
(
Tribs 🦇 ↓ dn ʎɐʍ sᴉɥʇ ↓🦇,
Sat 4 Jul 2015, 9:22,
archived)