In Greek mythology Cerberus is the terrifying, three-headed dog who guards the entrance to Hades. He is the offspring of Typhoeus, a fire-breathing giant, and Echidna, a half-woman, half-snake monster. Web resources here and here.

Hercules, Cerberus and Mercury. From the Mausoleum of Cerberus, Giugliano, Italy. Fresco. Roman Republican or Early Imperial (first c. BCE–first c. CE). Photo: Soprintendenza Archeologia Belle Arti e Paesaggio per l’Area Metropolitana di Napoli.