This mythelic creature gaurded the city thebesshe posed a riddle to everyone who passed her immediately killed anyone who could answer it name the monster?

The creature you're referring to is the Sphinx. In Greek mythology, the Sphinx guarded the city of Thebes and posed a riddle to travelers, killing anyone who could not answer correctly. The famous riddle was, "What walks on four legs in the morning, two legs at noon, and three legs in the evening?" Oedipus ultimately solved the riddle, leading to the Sphinx's defeat.