The term 'many-headed serpent' is often used to refer to the mythical creature known as the HYDRA. The HYDRA was a giant serpent-like monster in Greek mythology with multiple heads, and it was said that when one of its heads was cut off, two more would grow in its place. It was eventually killed by Hercules as part of his twelve labors. In crossword puzzles, the term 'many-headed serpent' is often abbreviated to 'HYDRA, ' which is a five-letter word.