I think everybody knows him ! In Greek Mythology, he is the son of the titan Typhon and the female monster Echidna. He is Hades' dog who made him the guardian of the Underworld, who must let no living get in and no dead get out, who got fooled only 3 times or 4 times if we count Heracles knocking him out. He is a gigantic 3 headed dog as you know it, but also with a dragon tail, his back scattered in snakes and most likely with spots (Cerberus means spot. Hades is not original) He is ferocious but acts like a normal dog, loving sweets and make his owner happy.

Cerberus © Greek mythology
Drawing © me