In Hindu mythology, the demon defeated by the goddess Durga is Mahishasura. This buffalo-headed demon posed a significant threat to the gods and humanity by conquering the heavens and disrupting cosmic order. In response, the gods created Durga, a powerful goddess embodying divine energies to combat him. The battle between Durga and Mahishasura symbolizes the triumph of good over evil and is celebrated during the festival of Durga Puja, highlighting the strength and resilience found in devotion and faith.