Shelley was actually Toshiaki's dead turtle. When he digs up her corpse to use Victor Frankenstein's reanimation secrets on it, the experiment goes awry and the resurrected turtle grows to an extremely tremendous size. She is defeated and turned back to her original, dead state when she steps in an electrocuted puddle of beer and falls back onto the carnival's ferris wheel.
Shelley's appearance is based on Gamera, a Japanese giant turtle kaiju similar to Godzilla known by the title of "Friend of All Children", with Shelley's malevolent behavior being a deliberate contrast to this identity.
The Turtle Monster's death of being electrocuted on a ferris wheel resembles the death of the Rhedosaurus in a burning Coney Island roller coaster in the ending of The Beast from 20,000 Fathoms.
Turtle Monster's name is based on the author's surname of Frankenstein, Mary Shelley.