The legendary beast said to have three heads is the Cerberus. In Greek mythology, Cerberus is known as the hound of Hades, guarding the entrance to the Underworld. His three heads symbolize the past, present, and future. Cerberus prevents the dead from escaping and the living from entering without permission, embodying a crucial aspect of the afterlife's boundaries. This creature has been featured in numerous myths and artistic depictions, illustrating the connection between life and death within ancient Greek culture.