There's no one definitive answer to this question - it depends on who you ask! Some people believe that the first monster was created by Mary Shelley in her 1818 novel Frankenstein. Others argue that monsters have been around for much longer, appearing in myths and legends from cultures all over the world.
Frankenstein is often considered the first true monster story, and its titular character has become an icon of horror. The novel tells the story of scientist Victor Frankenstein, who creates a creature from stolen body parts and brings it to life using electricity. The creature is initially happy to be alive, but soon realizes that he is hideous and rejected by society. He turns on his creator, leading to a tragic ending for both Frankenstein and his monster.