Frankenstein's Monster is based on the character of the same name from Mary Wollstonecraft Shelley's book Frankenstein; or, The Modern Prometheus. It was a creature made from corpse matter and mad science by one Victor Frankenstein.
The visual appearance (and often characterization) of the monster is often based on the classic portrayal by Boris Karloff in the 1931 Universal film by James Whale and its sequels.
A character modelled after Frankenstein's Monster is seen in a crowd shot in this film at the end of the "This is Halloween" musical number. He is seen applauding with the other citizens celebrating Halloween when Jack Skellington appears.
In the show Once Upon a Time, Frankenstein's Monster is reimagined as the character of Gerhardt Frankenstein. Here, he was the brother of Victor Frankenstein who assisted him in his scientific experiments. However, while helping Victor to graverob he would be mortally shot by a police officer.
Following this, Victor became obsessed with trying to ressurect his brother rather than create life. This lead to Victor being confronted by Rumplestiltskin who offered him a deal; exchanging a magic infused heart for Gerhardt's ressurection in return for Victor to fake failing to ressurect Regina's lover Daniel. After doing this, Victor was able to ressurect Gerhardt albeit as a monster who murdered their own father.
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